Engilis v. Monsanto Company

FOR PUBLICATION

UNITED STATES COURT OF APPEALS

FOR THE NINTH CIRCUIT

PETER ENGILIS, Jr.; CATHY No. 23-4201

ENGILIS,

D.C. No.

3:19-cv-07859-

Plaintiffs - Appellants,

VC

v.

MONSANTO COMPANY, OPINION

Defendant - Appellee.

Appeal from the United States District Court

for the Northern District of California

Vince Chhabria, District Judge, Presiding

Argued and Submitted March 7, 2025

Pasadena, California

Filed August 12, 2025

Before: Richard C. Tallman, Richard R. Clifton, and

Morgan B. Christen, Circuit Judges.

Opinion by Judge Christen

2 ENGILIS V. MONSANTO COMPANY

SUMMARY*

Expert Testimony

Affirming the district court’s summary judgment in

favor of Monsanto Company in multidistrict litigation

concerning Monsanto’s glyphosate-based herbicide

Roundup, the panel held that the district court did not abuse

its discretion in excluding the opinion of an expert witness

that exposure to Roundup likely caused Peter Engilis’s blood

cancer.

To establish causation, expert witness Dr. Andrew

Schneider conducted a differential etiology, which is an

established scientific technique for establishing the cause of

a medical condition. The district court concluded that the

expert’s differential etiology was unreliable pursuant to Fed.

R. Evid. 702 because the expert failed to reliably rule out

obesity as a potential cause of Engilis’s cancer.

A proponent of expert testimony must always establish

the admissibility criteria of Rule 702 by a preponderance of

the evidence. There is no presumption in favor of admission.

The panel rejected Engilis’s contention that Dr.

Schneider adequately supported his assertion that Engilis

was not obese. Aside from the reference to Engilis’s fact

sheet, Dr. Schneider’s expert report provided no support for

his conclusion that Engilis was not obese. At the Daubert

hearing, Dr. Schneider conceded that he could not say

whether Engilis was obese or not.

*

This summary constitutes no part of the opinion of the court. It has

been prepared by court staff for the convenience of the reader.

ENGILIS V. MONSANTO COMPANY 3

The panel also rejected Engilis’s contention that the

district court erred by discounting Dr. Schneider’s clinical

experience and overlooking that the weight of scientific

literature had found no positive association between obesity

and non-Hodgkin’s lymphoma (NHL). Contrary to the body

of scientific literature, Dr. Schneider testified on cross-

examination, that, in his view, obesity was not a risk factor

for NHL or chronic lymphocytic leukemia. Without

articulating a reasoned basis for his opinion, Dr. Schneider

failed to establish that his testimony was “based on sufficient

facts or data.” Fed. R. Evid. 702(b).

Because Dr. Schneider’s excluded opinion was the sole

evidence upon which Engilis relied to establish causation,

the panel affirmed the district court’s order granting

summary judgment in favor of Monsanto.

COUNSEL

Thomas A. Burns (argued), Burns PA, Tampa, Florida;

Jeffrey L. Haberman, Schlesinger Law Offices PA, Fort

Lauderdale, Florida; Bryan S. Gowdy, Creed & Gowdy,

Jacksonville, Florida; for Plaintiffs-Appellants.

Nicole Antoine (argued), David M. Zionts, Madison L.

Ferris, and Michael X. Imbroscio, Covington & Burling

LLP, Washington, D.C.; Brian L. Stekloff, Wilkinson

Stekloff LLP, Washington, D.C.; Jed P. White, Bryan Cave

Leighton Paisner LLP, Santa Monica, California; K. Lee

Marshall, Bryan Cave Leighton Paisner LLP, San Francisco,

California; Kathryn Podsiadlo, Arnold & Porter Kaye

Scholer LLP, Los Angeles, California; for Defendant-

Appellee.

4 ENGILIS V. MONSANTO COMPANY

OPINION

CHRISTEN, Circuit Judge:

This appeal arises from the long-running multidistrict

litigation concerning Monsanto’s glyphosate-based

herbicide called Roundup. Plaintiffs Peter Engilis, Jr. and

Cathy Engilis challenge the district court’s order excluding

their expert witness’s opinion that exposure to Roundup

likely caused Peter Engilis’s blood cancer. To establish

causation, the expert conducted a differential etiology.

A differential etiology is an established scientific

technique for establishing the cause of a medical condition.1

This technique is generally accomplished by first

determining (or, “ruling in”) “all of the potential hypotheses

that might explain a patient’s symptoms,” and then

eliminating (or, “ruling out”) potential hypotheses “on the

basis of a continuing examination of the evidence so as to

reach a conclusion as to the most likely cause of the findings

in that particular case.” Clausen v. M/V New Carissa, 339

F.3d 1049, 1057–58 (9th Cir. 2003). Courts, including our

own, have generally recognized that a sufficiently reliable

1

Although some cases have used the terms “differential etiology” and

“differential diagnosis” interchangeably, the methodology is “more

accurately referred to as differential etiology.” Fed. Jud. Ctr., Reference

Manual on Scientific Evidence 617 (3d ed. 2011). In the clinical context,

“differential diagnosis” refers to a process for “identifying a set of

diseases or illnesses responsible for the patient’s symptoms,” whereas

“‘differential etiology’ refers to identifying the causal factors involved

in an individual’s disease or illness.” Id. at 617 n.211. Put another way,

“differential diagnosis actually refers to a method of diagnosing an

ailment, not determining its cause,” and “differential etiology . . . is a

causation-determining methodology.” Higgins v. Koch Dev. Corp., 794

F.3d 697, 705 (7th Cir. 2015) (citation modified).

ENGILIS V. MONSANTO COMPANY 5

differential etiology “may form the basis of an expert’s

causation testimony.” Messick v. Novartis Pharm. Corp.,

747 F.3d 1193, 1197 (9th Cir. 2014).

Here, the district court concluded the expert’s

differential etiology was unreliable pursuant to Federal Rule

of Evidence 702 because the expert failed to reliably rule out

obesity as a potential cause of Peter Engilis’s cancer. We

affirm.

I

From 1990 to 2015, Peter Engilis, Jr. 2 routinely hand-

sprayed Roundup several times per month at each of his

three homes in Florida. In 2014, he was diagnosed with a

blood cancer known as chronic lymphocytic leukemia

(CLL), which is a type of non-Hodgkin’s lymphoma (NHL).

In November 2019, Engilis filed a lawsuit against

Roundup manufacturer Monsanto in the Middle District of

Florida, invoking the court’s diversity jurisdiction and

asserting claims under Florida state law that were premised

on the allegation that exposure to Roundup caused him to

develop CLL. The case was subsequently transferred to a

multidistrict litigation proceeding in the Northern District of

California, in which thousands of cancer victims have

alleged that Roundup caused their NHL. See In re Roundup

Prod. Liab. Litig., No. 3:16-md-02741-VC (N.D. Cal.);

Hardeman v. Monsanto Co., 997 F.3d 941, 950 (9th Cir.

2021).3

2

Cathy Engilis is also a plaintiff in this action, but for present purposes,

we need refer only to Peter Engilis.

3

In Hardeman, we affirmed a judgment in favor of the plaintiff in the

first bellwether trial from the multidistrict litigation.

6 ENGILIS V. MONSANTO COMPANY

In a “toxic tort claim for physical injuries,” a plaintiff

must “show that he was exposed to chemicals that could

have caused the physical injuries he complains about

(general causation), and that his exposure did in fact result

in those injuries (specific causation).” Golden v. CH2M Hill

Hanford Grp., 528 F.3d 681, 683 (9th Cir. 2008). To

demonstrate that Roundup caused Engilis’s cancer, Engilis

relied on the expert opinion of board-certified oncologist Dr.

Andrew Schneider. Dr. Schneider submitted an expert

report offering opinions on both general causation and

specific causation, only the latter of which is at issue in this

appeal. To show specific causation—i.e., that exposure to

Roundup caused Engilis’s cancer—Dr. Schneider conducted

a differential etiology.

First, Dr. Schneider “ruled in” all potential causes of

Engilis’s cancer. To rule in Roundup as a possible cause, he

relied upon general causation experts who opined that

Roundup is capable of causing NHL and noted that Engilis

had extensive exposure to Roundup. He also ruled in other

risk factors, such as age, gender, ethnicity, geographic

location, family history, occupational and environmental

exposures, and various medical conditions. Second, he

“ruled out” numerous risk factors based on his examination

of the evidence. He ruled out, for example, other pesticides,

insecticides, and asbestos, because no evidence suggested

that Engilis was exposed to them.

Importantly, Dr. Schneider purported to rule out

numerous medical conditions, including obesity. Dr.

Schneider noted that, according to Engilis’s “Plaintiff Fact

Sheet,” Engilis was “negative” for obesity, along with nearly

ENGILIS V. MONSANTO COMPANY 7

twenty other medical conditions. 4 Dr. Schneider did not

discuss whether obesity or any of the other identified

conditions were capable of contributing to the development

of NHL. But because Engilis was “negative” for the listed

conditions, Dr. Schneider concluded that those conditions

were not “suggested as related to or as causative factors to

the onset of different types of cancers in Mr. Engilis’s case.”

Ultimately, Dr. Schneider opined that exposure to Roundup

caused or was a substantial factor in causing or contributing

to Engilis’s cancer.

Monsanto moved to exclude Dr. Schneider’s opinion.

The district court initially granted the motion in a brief order

without a hearing. Engilis sought reconsideration, and the

district court vacated its prior order to the extent it concerned

Dr. Schneider’s specific causation opinion and scheduled a

hearing.5

At the hearing on the motion to exclude, Monsanto’s

counsel extensively cross-examined Dr. Schneider about his

basis for ruling out Engilis’s obesity as a potential cause of

Engilis’s cancer. In response, Dr. Schneider sought to

defend his assertion that Engilis was not obese. But after

conceding that he had not examined Engilis and could not

say whether Engilis was obese or not, Dr. Schneider testified

4

The provenance of this fact sheet, including the identity of the person

who prepared it, is not clear from the record.

5

The district court has “broad latitude” to determine “the appropriate

form of the inquiry” under Rule 702. Est. of Barabin v. AstenJohnson,

Inc., 740 F.3d 457, 463 (9th Cir. 2014) (en banc), overruled in part on

other grounds by United States v. Bacon, 979 F.3d 766 (9th Cir. 2020)

(en banc). Although “pretrial ‘Daubert hearings’ are commonly

used, . . . they are certainly not required.” Id. (citation omitted); accord

United States v. Holguin, 51 F.4th 841, 852 (9th Cir. 2022).

8 ENGILIS V. MONSANTO COMPANY

that, regardless of whether Engilis was obese, he did not

view obesity as a potential cause of NHL. During follow-on

questioning, he stated that although some medical literature

reports an association between obesity and the development

of NHL, his clinical experience led him to believe that

obesity does not contribute to NHL.

After the hearing, the district court issued an order

excluding Dr. Schneider’s specific causation opinion. The

district court reasoned that Dr. Schneider did not reliably

rule out obesity as a potential cause of Engilis’s cancer, and

that this rendered his differential etiology unreliable.

Because the exclusion of Dr. Schneider’s testimony left

Engilis without evidence of specific causation, the district

court granted summary judgment in favor of Monsanto.

Engilis timely appealed.

II

We have jurisdiction pursuant to 28 U.S.C. § 1291. “We

review a district court’s decision to exclude expert testimony

for abuse of discretion.” Tekoh v. County of Los Angeles, 75

F.4th 1264, 1265 (9th Cir. 2023). We review an order

granting summary judgment de novo. Sonner v. Schwabe N.

Am., Inc., 911 F.3d 989, 992 (9th Cir. 2018).

III

A

The parties agree that the admissibility of expert

testimony is controlled by Federal Rule of Evidence 702.

See Elosu v. Middlefork Ranch Inc., 26 F.4th 1017, 1023 (9th

Cir. 2022). That Rule provides that, “before admitting

expert testimony, the district court must perform a

gatekeeping role to ensure that the [proffered] testimony is

both relevant and reliable.” United States v. Valencia-Lopez,

ENGILIS V. MONSANTO COMPANY 9

971 F.3d 891, 897–98 (9th Cir. 2020) (citation modified).

Generally, “[e]xpert opinion testimony is relevant if the

knowledge underlying it has a valid connection to the

pertinent inquiry” and “reliable if the knowledge underlying

it has a reliable basis in the knowledge and experience of the

relevant discipline.” Alaska Rent-A-Car, Inc. v. Avis Budget

Grp., Inc., 738 F.3d 960, 969 (9th Cir. 2013) (citation

omitted); see also Est. of Barabin v. AstenJohnson, Inc., 740

F.3d 457, 463 (9th Cir. 2014) (en banc), overruled in part on

other grounds by United States v. Bacon, 979 F.3d 766 (9th

Cir. 2020) (en banc).

However, the parties dispute the significance of the 2023

amendment to Rule 702 and the effect of that amendment on

our existing precedent. To address this issue, we briefly

recount the history of the Daubert trilogy, the amendments

to Rule 702, and our interpretation of that Rule in caselaw.

In Daubert v. Merrell Dow Pharmaceuticals, Inc., the

Supreme Court held that Rule 702 governs the admissibility

of expert testimony. 509 U.S. 579, 597 (1993). In so doing,

the Supreme Court firmly rejected the Frye “general

acceptance” test,6 which the Court described as inconsistent

“with the liberal thrust of the Federal Rules and their general

approach of relaxing the traditional barriers to opinion

testimony.” Id. at 588 (citation modified); see also Primiano

v. Cook, 598 F.3d 558, 564 (9th Cir. 2010).

6

In Frye v. United States, the D.C. Circuit excluded expert testimony

because the methodology at issue was not “sufficiently established to

have gained general acceptance in the particular field in which it

belong[ed].” 293 F. 1013, 1014 (D.C. Cir. 1923).

10 ENGILIS V. MONSANTO COMPANY

Rule 702 was first adopted in 1975 as part of the original

enactment of the Federal Rules of Evidence.7 At the time of

Daubert, Rule 702 provided that “[i]f scientific, technical, or

other specialized knowledge will assist the trier of fact to

understand the evidence or to determine a fact in issue, a

witness qualified as an expert by knowledge, skill,

experience, training, or education, may testify thereto in the

form of an opinion or otherwise.” Fed. R. Evid. 702 (1975).

Interpreting Rule 702, Daubert held that a district court

“[f]aced with a proffer of expert scientific testimony” must

“determine at the outset” by a preponderance of the

evidence, Fed. R. Evid. 104(a), that “the expert is proposing

to testify to (1) scientific knowledge that (2) will assist the

trier of fact to understand or determine a fact in issue.”

Daubert, 509 U.S. at 592 & n.10. As Daubert explained, the

Rule’s requirement that opinion testimony “assist the trier of

fact” “goes primarily to relevance,” and the Rule’s reference

to scientific “knowledge” demands a showing of reliability.

Id. at 590–91 (quoting Fed. R. Evid. 702). Thus, a district

court discharges its “gatekeeping role” under Rule 702 by

“ensur[ing] that any and all scientific testimony or evidence

admitted is not only relevant, but reliable.” Id. at 589, 597;

accord United States v. Ruvalcaba-Garcia, 923 F.3d 1183,

1188 (9th Cir. 2019).

After Daubert, the Supreme Court continued to refine its

interpretation of Rule 702 in General Electric Co. v. Joiner,

522 U.S. 136 (1997), and Kumho Tire Co. v. Carmichael,

526 U.S. 137 (1999). Joiner confirmed that in applying

abuse-of-discretion review to a district court’s ruling on the

admissibility of expert testimony, a court of appeals “may

7

See Act to Establish Rules of Evidence for Certain Courts and

Proceedings, Pub. L. No. 93-595, 88 Stat. 1926, 1937 (1975).

ENGILIS V. MONSANTO COMPANY 11

not categorically distinguish between rulings allowing

expert testimony and rulings disallowing it.” Joiner, 522

U.S. at 142. The Court also held that the reliability test may

be applied to an expert’s reasoning process. See id. at 146

(stating that “nothing . . . requires a district court to admit

opinion evidence that is connected to existing data only by

the ipse dixit of the expert” and that “[a] court may conclude

that there is simply too great an analytical gap between the

data and the opinion proffered”). In Kumho, the Court

clarified that the district court’s gatekeeper function applies

to all expert testimony, not just scientific expert testimony.

See Kumho, 526 U.S. at 141. Kumho also explained that the

reliability standard is applied flexibly, depending on the

relevant field of expertise. See id. at 149–50. Together,

these three decisions—Daubert, Joiner, and Kumho—

comprise the “Daubert trilogy.” Truck Ins. Exch. v.

MagneTek, Inc., 360 F.3d 1206, 1209 (10th Cir. 2004).

In 2000, Rule 702 was amended for the first time to

codify the holdings of the Daubert trilogy, and to resolve

conflicts that had arisen within the courts about the meaning

of that trilogy. Fed. R. Evid. 702 advisory committee’s note

to 2000 amendment; Fern M. Smith, Report of the Advisory

Committee on Evidence Rules 6–7 (1999) [hereinafter May

1, 1999 Report], https://perma.cc/LH3V-5GBB. The

amendment “clearly envision[ed] a more rigorous and

structured approach” to Rule 702 than some courts were then

employing. May 1, 1999 Report at 7. It “affirm[ed] the trial

court’s role as gatekeeper and provide[d] some general

standards that the trial court must use to assess the reliability

and helpfulness of proffered expert testimony.” Fed. R.

Evid. 702 advisory committee’s note to 2000 amendment.

To this end, the 2000 amendment added three reliability-

based requirements, now found in subsections (b), (c), and

12 ENGILIS V. MONSANTO COMPANY

(d). Thus, after a minor restyling amendment in 2011,8 Rule

702 provided that a qualified expert witness may testify if:

(a) the expert’s scientific, technical, or other

specialized knowledge will help the trier of

fact to understand the evidence or to

determine a fact in issue; (b) the testimony is

based on sufficient facts or data; (c) the

testimony is the product of reliable principles

and methods; and (d) the expert has reliably

applied the principles and methods to the

facts of the case.

Fed. R. Evid. 702 (2011). The Advisory Committee’s note

explained that the proponent of expert testimony “has the

burden of establishing that the pertinent admissibility

requirements are met by a preponderance of the evidence.”

Fed. R. Evid. 702 advisory committee’s note to 2000

amendment (citing Fed. R. Evid. 104(a)).

The 2000 amendment to Rule 702 did not eliminate

confusion and establish uniformity. Patrick J. Schiltz,

Report of the Advisory Committee on Evidence Rules, in

Comm. on Rules of Prac. & Proc., June 7, 2022 Agenda

Book 866, 871 (2022), https://perma.cc/AY5J-GAZA; see

also Thomas D. Schroeder, Toward A More Apparent

Approach to Considering the Admission of Expert

Testimony, 95 Notre Dame L. Rev. 2039, 2039–59 (2020).

The Rule was therefore amended again in December 2023 to

expressly require a proponent of expert testimony to

8

The 2011 amendment to Rule 702 sought “to make [it] more easily

understood and to make style and terminology consistent throughout the

rules,” and was not intended to implement any substantive changes. Fed.

R. Evid. 702 advisory committee’s note to 2011 amendment.

ENGILIS V. MONSANTO COMPANY 13

“demonstrate[] to the court that it is more likely than not

that” the four admissibility requirements are satisfied. Fed.

R. Evid. 702 (2023). The amendment also modified

subsection (d), which now requires that “the expert’s opinion

reflects a reliable application of the principles and methods

to the facts of the case.” Id.

This amendment sought to “clarify and emphasize” that

proffered expert testimony must meet the admissibility

requirements of Rule 702 by a preponderance of the

evidence. Fed. R. Evid. 702 advisory committee’s note to

2023 amendment. Before the amendment, “many courts”

had erroneously held “that the critical questions of the

sufficiency of an expert’s basis, and the application of the

expert’s methodology, are questions of weight and not

admissibility.” Id. Properly applied, Rule 702 requires that

challenges to an expert’s opinion go to the weight of the

evidence only if a court first finds it more likely than not that

an expert has a sufficient basis to support an opinion. Id.

The amendment also aimed “to emphasize that each expert

opinion must stay within the bounds of what can be

concluded from a reliable application of the expert’s basis

and methodology.” Id. “Judicial gatekeeping is essential”

to ensure that an expert’s conclusions do not “go beyond

what the expert’s basis and methodology may reliably

support.” Id. As the Advisory Committee’s note explains,

the amendment did not “impose[] any new, specific

procedures,” and was “simply intended to clarify” existing

law. Id.

For present purposes, we need not undertake an

exhaustive examination of the effects, if any, of the 2023

14 ENGILIS V. MONSANTO COMPANY

amendment on our caselaw.9 But insofar as the parties argue

about the degree to which the amendments establish, or

refute, that Rule 702 is a “liberal” standard that favors

admission as “the rule, not the exception,” we confirm that a

proponent of expert testimony must always establish the

admissibility criteria of Rule 702 by a preponderance of the

evidence and that there is no presumption in favor of

admission.

Our precedent has long recognized the burden-of-proof

principles that the amendment sought to clarify. We have

expressly held that the “preliminary questions” of Rule 702

“must be established by a preponderance of proof.” See

United States v. Rincon, 28 F.3d 921, 923 (9th Cir. 1994);

see also United States v. Evans, 728 F.3d 953, 960 n.6 (9th

Cir. 2013). This is unsurprising, as Daubert itself held that

the requirements of relevance and reliability must be

“established by a preponderance of proof” pursuant to Rule

104(a). Daubert, 509 U.S. at 592 n.10.

Several of our cases have stated that “Rule 702 should

be applied with a ‘liberal thrust’ favoring admission.”

Messick, 747 F.3d at 1196 (quoting Daubert, 509 U.S. at

588); Wendell v. GlaxoSmithKline LLC, 858 F.3d 1227,

1232 (9th Cir. 2017). Rule 702 liberalized the admission of

expert testimony as compared to the Frye test, but it did not

9

Because the district court resolved the motion to exclude two weeks

before the 2023 amendment went into effect on December 1, 2023, it

applied the prior version of Rule 702. On appeal, we apply the version

of Rule 702 in effect at the time of the district court’s ruling. See

Barabin, 740 F.3d at 463 & n.4; see also D’Pergo Custom Guitars, Inc.

v. Sweetwater Sound, Inc., 111 F.4th 125, 140 n.11 (1st Cir. 2024); In re

Onglyza (Saxagliptin) & Kombiglyze (Saxagliptin & Metformin) Prods.

Liab. Litig., 93 F.4th 339, 345 n.4 (6th Cir. 2024). But we note that our

decision would be the same under either version of the Rule.

ENGILIS V. MONSANTO COMPANY 15

establish a categorical preference for admitting expert

testimony. See Joiner, 522 U.S. at 142 (emphasizing a

district court’s “gatekeeper” role and holding that a court of

appeals “may not categorically distinguish between rulings

allowing expert testimony and rulings disallowing it”); see

also Daubert, 509 U.S. at 588. Although Rule 702 “allow[s]

district courts to admit a somewhat broader range of

scientific testimony than would have been admissible under

Frye, [it] leave[s] in place the ‘gatekeeper’ role of the trial

judge in screening such evidence.” Joiner, 522 U.S. at 142;

see also Weisgram v. Marley Co., 528 U.S. 440, 455 (2000)

(noting that parties seeking to introduce expert testimony

must meet “exacting standards of reliability”). Our caselaw

should not be understood to suggest a presumption of

admission. There is no such presumption, as a proponent of

expert testimony must always establish the admissibility

requirements of Rule 702 by a preponderance of the

evidence. See Fed. R. Evid. 702 (2023).

We have also stated that, where experts’ opinions “are

not the ‘junk science’ Rule 702 was meant to exclude,”

Wendell, 858 F.3d at 1237 (citation omitted), “the interests

of justice favor leaving difficult issues in the hands of the

jury and relying on the safeguards of the adversary

system . . . to ‘attack[] shaky but admissible evidence,’” id.

(quoting Daubert, 509 U.S. at 596). To be sure, Rule 702 is

concerned with “the soundness of [the expert’s]

methodology,” rather than “the correctness of the expert’s

conclusions.” Primiano, 598 F.3d at 564 (citation omitted).

But “shaky” expert testimony, like any expert testimony,

must still be “admissible,” and this requires a determination

by the trial court that it satisfies the threshold requirements

established by Rule 702. See Wendell, 858 F.3d at 1237

(citation omitted); accord Messick, 747 F.3d at 1196. Only

16 ENGILIS V. MONSANTO COMPANY

“[i]f the proposed testimony meets the thresholds of

relevance and reliability” is its proponent “entitled to have

the jury decide upon its credibility.” Elosu, 26 F.4th at 1024

(citation modified). The district court “cannot abdicate its

role as gatekeeper,” nor “delegat[e] that role to the jury.”

Hardeman, 997 F.3d at 960 n.11 (citation omitted).

Consistent with the 2023 amendment, our precedent

establishes that Rule 702 requires a proponent of expert

testimony to demonstrate each of the requirements of Rule

702 by a preponderance of the evidence.

B

With that historical background, we now turn to the case

before us. The district court excluded Dr. Schneider’s

causation opinion as unreliable pursuant to Rule 702 because

Dr. Schneider failed to follow the differential etiology

methodology his report purported to employ and failed to

reliably rule out obesity as a potential cause of Engilis’s

cancer. In the district court’s view, Dr. Schneider’s

testimony was flawed in two ways. First, Dr. Schneider

asserted that Engilis was not obese and relied on this

assertion of fact in his analysis, but failed to justify it.

Second, Dr. Schneider pivoted on cross-examination and

attempted to argue that obesity is not actually a risk factor

for NHL, but he also failed to justify that position. We

address each of these issues in turn.

1

Engilis argues Dr. Schneider adequately supported his

assertion that Engilis was not obese. In Engilis’s view, the

district court erred because it overlooked genuine disputes of

material fact about Engilis’s weight and supposed obesity.

We disagree.

ENGILIS V. MONSANTO COMPANY 17

The district court’s “responsibility to screen expert

testimony,” Elosu, 26 F.4th at 1020, encompasses the

requirement that expert testimony be “based on sufficient

facts or data,” Fed. R. Evid. 702(b). This element “requires

foundation.” Elosu, 26 F.4th at 1025. The “key inquiry” is

“whether an expert had sufficient factual grounds on which

to draw conclusions.” Hyer v. City & County of Honolulu,

118 F.4th 1044, 1056 (9th Cir. 2024) (citation omitted).

Dr. Schneider expressly asserted in his expert report that

Engilis was not obese and relied on this proposition to form

his differential etiology opinion. The report states that

Engilis’s “Plaintiff Fact Sheet” indicates Engilis is

“negative” for several medical conditions, including obesity.

At the outset of his analysis, Dr. Schneider therefore ruled

out obesity as a causative factor in the onset of Engilis’s

cancer.

Aside from the reference to Engilis’s fact sheet, Dr.

Schneider’s expert report provides no support for his

conclusion that Engilis was not obese. The expert report

does not cite any of Engilis’s medical records to support the

conclusion that he was not obese. It does not reference

Engilis’s weight, BMI,10 or body shape. And it does not cite

or rely on any testimony from Engilis.

At the Daubert hearing, counsel for Monsanto cross-

examined Dr. Schneider about the basis for his opinion that

Engilis was not obese. Counsel presented Dr. Schneider

10

“BMI” is an acronym for “body mass index” and is a measure of

weight in relation to height. Nat’l Heart, Lung, & Blood Inst., Clinical

Guidelines on the Identification, Evaluation, and Treatment of

Overweight and Obesity in Adults: The Evidence Report xiv (1998). As

Dr. Schneider acknowledged at the Daubert hearing, a BMI over 30

qualifies as obese.

18 ENGILIS V. MONSANTO COMPANY

with medical records suggesting that, contrary to Dr.

Schneider’s assertion in his report, Engilis may have been

obese. For instance, one record from December 2000

showed Engilis then had a BMI of 32.9. And according to

another record, Engilis had a BMI of 33 in November 2014.

Confronted with these records, Dr. Schneider countered that

a diagnosis of obesity is more complex than merely

measuring BMI, as BMI does not account for the distribution

of fat on a person’s body, which he regarded as the most

salient consideration in determining obesity. But Dr.

Schneider admitted that he did not know how Engilis’s

weight was distributed, and that he had not ever spoken to

Engilis or seen a photo of him. Ultimately, Dr. Schneider

conceded that he “certainly can’t say whether [Engilis is]

obese or not.”

To the extent Dr. Schneider testified on cross-

examination about the shortcomings of BMI as a measure of

obesity, this opinion was inadmissible for a separate reason:

it was not disclosed in his expert report. See Fed. R. Civ. P.

26(a)(2)(B). An expert’s written report must contain, among

other things, “a complete statement of all opinions the

witness will express and the basis and reasons for them,” and

“the facts or data considered by the witness in forming

them.” Fed. R. Civ. P. 26(a)(2)(B)(i)-(ii). The federal rules

generally “forbid[] the use of any information not properly

disclosed.” Key v. Qualcomm Inc., 129 F.4th 1129, 1143

(9th Cir. 2025). Thus, “when a party fails to provide

information required by Rule 26, such party ‘is not allowed

to use that information or witness to supply evidence on a

motion, at a hearing, or at a trial, unless the failure was

substantially justified or is harmless.’” Id. (quoting Fed. R.

Civ. P. 37(c)(1)).

ENGILIS V. MONSANTO COMPANY 19

Dr. Schneider’s report does not mention BMI at all, let

alone describe its limitations. Moreover, although Dr.

Schneider testified that fat distribution is the proper measure

of obesity, he did not make that assertion—or even mention

fat distribution—in his report. 11 And even if he had, Dr.

Schneider admitted at the Daubert hearing that he had not

undertaken any evaluation of Engilis’s weight—using BMI,

fat distribution, or any other metric. Indeed, in contravention

of the assertion made in his report, Dr. Schneider conceded

that he could not say whether Engilis was obese or not.

Ultimately, the critical question for purposes of the

court’s gatekeeping function is not whether Engilis was

actually obese or what inferences about obesity the record

might support. Instead, the issue is whether Dr. Schneider’s

opinion that Engilis was not obese is “based on sufficient

facts or data.” See Fed. R. Evid. 702(b). For that inquiry,

what matters is the evidence Dr. Schneider actually

considered and the conclusions he actually drew from that

evidence in the process of forming his opinion as disclosed

in his expert report. Here, the district court properly

concluded that Engilis failed to establish by a preponderance

of the evidence that Dr. Schneider’s conclusion was based

on sufficient facts or data.

11

Engilis also seeks to support Dr. Schneider’s conclusion that Engilis

was not obese with his own deposition testimony about his weight.

Engilis notes that he testified he was roughly 170 pounds in 1990, and

slowly put on weight over the years until he was roughly 195 pounds at

the time of his deposition in 2021. Engilis argues that “simple

arithmetic” suggests a linear weight gain of about 0.8 pounds per year,

and that under this rate, he would not have become obese until 2010. But

Dr. Schneider did not conduct any of this analysis in his report.

Moreover, even if the analysis was proper and disclosed, it suggests that

Engilis was obese before his CLL diagnosis in 2014.

20 ENGILIS V. MONSANTO COMPANY

2

Engilis next argues the district erred by concluding that

Dr. Schneider failed to reliably support his conclusion that,

even if Engilis was obese, he properly ruled out obesity

because it is not a risk factor for NHL. More specifically,

Engilis contends that the district court erred by discounting

Dr. Schneider’s clinical experience and overlooking that the

weight of scientific literature has found no positive

association between obesity and NHL. We reject Engilis’s

position.

As explained, a differential etiology “starts with ruling

in all potential causes, then ruling out the ones as to which

there is no plausible evidence of causation, and then

determining the most likely cause among those that cannot

be excluded.” Hardeman, 997 F.3d at 965 (citation

modified). Dr. Schneider’s expert report did not discuss

whether obesity is properly recognized as a potential cause

of NHL. Instead, Dr. Schneider simply concluded that

obesity could be ruled out as a potential cause because,

according to Engilis’s “fact sheet,” Engilis was “negative”

for obesity. Because Dr. Schneider’s report did not grapple

with the relationship between obesity and NHL, it did not

cite or describe any medical literature or studies on that

relationship.

At the Daubert hearing, after counsel cross-examined

Dr. Schneider about his basis for asserting that Engilis was

not obese, Dr. Schneider pivoted to contesting that obesity is

capable of causing NHL. During questioning, Dr. Schneider

conceded that obesity is recognized in medical literature as

a possible risk factor for NHL and testified that he had seen

articles on the association between obesity and NHL

throughout his career. He further stated that he had reviewed

ENGILIS V. MONSANTO COMPANY 21

papers on the topic and accepted that at least some of them

reported a positive association between obesity and the

development of NHL.

Contrary to this body of scientific literature, Dr.

Schneider testified on cross-examination that, in his view,

obesity is not a specific risk factor for NHL or CLL. Based

on his 34 years of practicing oncology, he stated that he

personally saw no association between weight and CLL and

that he saw “fat people get CLL” and saw “skinny people get

CLL.” Dr. Schneider also referred to an article he found on

Google the night before the Daubert hearing, which

purportedly showed that, of forty studies that examined the

relationship between obesity and NHL, only three showed a

positive association. As to Engilis specifically, Dr.

Schneider explained that he considered his BMI as a

potential cause, but “rejected it.”

Engilis insists that the article Dr. Schneider mentioned at

the Daubert hearing shows that Dr. Schneider properly ruled

out obesity as a risk factor. But this article cannot salvage

Dr. Schneider’s opinion because, irrespective of that

article’s conclusions, Dr. Schneider did not cite it or any

other relevant literature in his report. Without a citation to

scientific literature in Dr. Schneider’s report, Engilis cannot

now claim that scientific literature is the basis for Dr.

Schneider’s opinion. See Fed. R. Civ. P. 26(a)(2)(B).

Engilis also argues that, by excluding Dr. Schneider’s

opinion, the district court improperly dismissed Dr.

Schneider’s decades of education, training, and clinical

experience with NHL and CLL as a board-certified

oncologist. To be sure, an expert’s specialized knowledge

and experience is of critical significance when the district

court determines whether a witness is “qualified as an

22 ENGILIS V. MONSANTO COMPANY

expert.” Fed. R. Evid. 702. Importantly, such knowledge

and experience can also “serve as the requisite ‘facts or data’

on which they render an opinion.” Elosu, 26 F.4th at 1024.

Because “‘medical knowledge is often uncertain’ due to the

complexity of the human body and the novelty of emerging

medical issues,” we have noted that “‘physicians must use

their knowledge and experience as a basis for weighing

known factors along with the inevitable uncertainties’ to

make ‘a sound judgment’ in each case.” Id. at 1025 (quoting

Primiano, 598 F.3d at 565–66). Accordingly, “when an

expert establishes causation based on a differential

[etiology], the expert may rely on his or her extensive

clinical experience as a basis for ruling out a potential cause

of the disease.” Wendell, 858 F.3d at 1237.

But when ruling out potential causes, an expert “must

provide reasons for rejecting alternative hypotheses ‘using

scientific methods and procedures’” and must rely on “more

than ‘subjective beliefs or unsupported speculation.” See

Clausen, 339 F.3d at 1058 (quoting Claar v. Burlington N.

R.R. Co., 29 F.3d 499, 502 (9th Cir. 1994)); accord Messick,

747 F.3d at 1198. And the expert must do so in their expert

report. Fed. R. Civ. P. 26(a)(2)(B)(i)-(ii). Here, aside from

Dr. Schneider’s conclusory assertion of his subjective

opinion at the Daubert hearing, he provided no explanation

for ruling out obesity as a possible risk factor. Without

articulating a reasoned basis for his opinion, Dr. Schneider

failed to establish that his testimony was “based on sufficient

facts or data.” Fed. R. Evid. 702(b).

Our decisions in Messick, Wendell, and Hardeman, upon

which Engilis relies, do not hold otherwise. In each of these

cases, experts grounded their opinion in existing scientific

literature and studies, however limited, as well as their

clinical experience. In Messick, we noted “there is nothing

ENGILIS V. MONSANTO COMPANY 23

wrong with a doctor relying on extensive clinical experience

when making a differential [etiology],” but the expert there

also relied on existing medical literature to form his opinion.

Messick, 747 F.3d at 1198. Moreover, the expert described

his clinical experience in detail, opining that—based on his

extensive experience diagnosing and treating patients with

osteonecrosis of the jaw (ONJ)—a patient without cancer or

radiation exposure would not develop long-term ONJ unless

they received intravenous bisphosphonate treatments. Id. In

Wendell, we considered opinions from doctors “at or near

the top of their field” who had “extensive clinical

experience” with the rare disease at issue in that case.

Wendell, 858 F.3d at 1237. The relative dearth of medical

literature addressing causation for the exceedingly rare

cancer at issue demanded some reliance on clinical

experience, but the doctors nevertheless also consulted and

relied upon the studies and literature that did exist. See id.

at 1236. Hardeman concluded that, like the experts in

Messick who “relied on clinical experience as well as an

examination of medical literature and plaintiff’s records,”

the plaintiff’s experts relied on “epidemiological, animal,

and cell studies.” Hardeman, 997 F.3d at 963.

None of these cases stands for the proposition that an

expert’s mere talismanic invocation of “clinical experience”

suffices to establish that a differential etiology passes muster

under Rule 702. Instead, they establish that clinical

experience may be one basis on which an expert supports

their analysis. See Wendell, 858 F.3d at 1237; Elosu, 26

F.4th at 1024. We also note that, given the flexibility of the

reliability inquiry, Valencia-Lopez, 971 F.3d at 898, clinical

experience that might constitute “sufficient facts or data” in

one case, Fed. R. Evid. 702(b), might not suffice in another.

For example, reliance on extensive clinical experience might

24 ENGILIS V. MONSANTO COMPANY

be particularly informative—and perhaps necessary—in the

context of a rare disease. But it might be less probative in

the context of a more common disease for which there exists

a substantial body of established literature.

Unlike in Messick, Wendell, and Hardeman, the expert

report in this case does not describe the details of its author’s

clinical experience, nor does it include an assessment of the

existing literature. The opinion lacks the required reasoned

explanation for the assertion that obesity is not a risk factor

for NHL, and for Dr. Schneider’s decision to set aside, sub

silentio, the corpus of medical literature addressing a

relationship between obesity and NHL.

Engilis also contends that Dr. Schneider was not required

to rule out obesity as a possible cause because he need only

have ruled out “obvious” causes. For support, Engilis notes

that, to assess whether an expert’s testimony is sufficiently

reliable, courts may consider “[w]hether the expert has

adequately accounted for obvious alternative explanations.”

Fed. R. Evid. 702 advisory committee’s note to 2000

amendment. We are not persuaded.

Engilis is correct that “we have consistently recognized

the difficulties in establishing certainty in the medical

sciences.” Messick, 747 F.3d at 1198; see also Daubert, 509

U.S. at 590 (noting “there are no certainties in science”).

Given this uncertainty, “[w]e do not require experts to

eliminate all other possible causes of a condition for the

expert’s testimony to be reliable.” Wendell, 858 F.3d at

1237. To meet the threshold of reliability, the expert need

not purport to conclusively “identify the sole cause of a

medical condition.” Messick, 747 F.3d at 1199. But where

an expert rules out a potential cause, the expert “must

provide scientifically sound reasons” for doing so. Id. at

ENGILIS V. MONSANTO COMPANY 25

1198; accord Clausen, 339 F.3d at 1058. Here, Dr.

Schneider did not assert that he was unable to rule out

obesity or other possible causes. See Wendell, 858 F.3d at

1237 (describing “the alleged inability of the experts to rule

out an idiopathic origin” or another possible cause). Nor did

Dr. Schneider opine that he could not determine which of

multiple risk factors caused the condition at issue. See

Messick, 747 F.3d at 1199. Instead, Dr. Schneider rejected

obesity as a possible cause, but inadequately explained his

reasons for doing so. Accordingly, the district court properly

concluded that Engilis failed to establish by a preponderance

of the evidence that Dr. Schneider’s conclusion was based

on sufficient facts or data. See Fed. R. Evid. 702(b). In

doing so, the court properly exercised its gatekeeping

function.

IV

The district court did not abuse its discretion by granting

Monsanto’s motion to exclude.12 Because Dr. Schneider’s

excluded opinion was the sole evidence upon which Engilis

relied to establish causation, we affirm the district court’s

order granting summary judgment in favor of Monsanto.

AFFIRMED.

12

Because we affirm the district court’s exclusion of Dr. Schneider’s

opinion, we need not consider Monsanto’s alternative argument that

Engilis’s claims are preempted by federal law.