## Facts

- Thousands of plaintiffs in a federal MDL alleged that Monsanto’s glyphosate-based herbicide, Roundup, caused their non-Hodgkin lymphoma (NHL) and that Monsanto failed to warn of the risk.
- The order concerned several bellwether plaintiffs who reported heavy, long-term Roundup use followed by NHL diagnoses.
- Plaintiffs offered specific-causation opinions from treating and retained physicians who applied differential diagnosis (differential etiology) to conclude Roundup was a substantial factor in causing each plaintiff’s NHL.
- The experts “ruled in” Roundup based on (i) admissible general-causation evidence that glyphosate can cause NHL in some humans and (ii) each plaintiff’s exposure history.
- The experts “ruled out” alternative explanations by evaluating medical records and known risk factors and explaining why they were unlikely causes for each plaintiff.
- Monsanto moved to exclude plaintiffs’ specific-causation experts under Federal Rule of Evidence 702 and *Daubert* and sought summary judgment, arguing plaintiffs could not prove causation without admissible expert testimony.

## Issues

1. Whether plaintiffs’ specific-causation experts used a reliable methodology under Rule 702 and *Daubert*—particularly in “ruling in” Roundup and “ruling out” alternative and idiopathic causes—so their opinions were admissible.
2. Whether Monsanto was entitled to summary judgment on specific causation if plaintiffs lacked admissible expert testimony that Roundup was a substantial factor in causing their NHL.

## Decision

- The court denied Monsanto’s *Daubert* motions to exclude the plaintiffs’ specific-causation experts.
- The court held that differential diagnosis is an accepted method for specific causation and was reliably applied to the bellwether plaintiffs.
- The court rejected exclusion arguments based on the absence of a biomarker or epidemiological threshold unique to Roundup-related NHL, treating those criticisms as weight rather than admissibility.
- The court rejected Monsanto’s contention that the experts had to definitively exclude unknown (idiopathic) causes to satisfy Rule 702.
- The court denied Monsanto’s motion for summary judgment because admissible expert testimony created triable disputes on specific causation.

## Legal Principles

- Differential diagnosis is a generally accepted methodology for specific-causation opinions in toxic-tort litigation when the expert systematically identifies plausible causes and explains why alternatives are less likely in the individual case.
- A specific-causation expert may “rule in” a suspected agent by relying on admissible general-causation evidence together with patient-specific exposure and medical history; the lack of a condition-specific biomarker does not alone render the opinion inadmissible.
- Rule 702 does not require an expert to eliminate every conceivable cause, including unknown idiopathic causes; the expert must reasonably address known risk factors and explain why the proposed cause is the most plausible based on the record and scientific evidence.
- Courts assess the reliability of the expert’s methodology and its application, while disputes over the strength of supporting studies, exposure quantification, and competing interpretations are ordinarily for cross-examination and contrary evidence.
- When specific-causation expert testimony is admissible, summary judgment is generally improper if that testimony creates a genuine dispute of material fact on causation.

## Conclusion

The court permitted bellwether plaintiffs to present specific-causation opinions based on differential diagnosis and denied Monsanto summary judgment, holding that challenges centered on competing interpretations of scientific and patient-specific evidence largely concerned evidentiary weight for the jury, not admissibility under Rule 702 and *Daubert*.


In re Roundup Prods. Liab. Litig., 358 F. Supp. 3d 956 (N.D. Cal. 2019)