A federal judge on Monday waded into the arcane science behind claims that the widely used weed killer Roundup can cause cancer. The expected weeklong testimony is intended to help him determine whether a jury should hear from doctors who link the product to non-Hodgkin’s lymphoma.

U.S. District Judge Vince Chhabria heard from an epidemiologist at the University of California, Los Angeles about how she evaluated scientific studies of glyphosate, the active ingredient in Roundup, to arrive at her conclusion that it can cause cancer.

“After reviewing all of the scientific literature at hand, I really concluded that to a reasonable scientific degree of certainty, glyphosate and glyphosate based compounds, including Roundup, do indeed cause NHL,” the epidemiologist, Beate Ritz, said.

Many regulators have rejected a link between Roundup and cancer. Monsanto vehemently denies it, saying hundreds of studies have found glyphosate is safe. Monsanto will call its own experts later in the week to defend glyphosate.

Chhabria is presiding over more than 300 lawsuits against Monsanto Co. by cancer victims and their families who say the company long knew about Roundup’s cancer risk but failed to warn them.

The plaintiffs must first persuade Chhabria, however, that the claim that glyphosate can cause cancer has been tested, reviewed and published and is widely accepted in the scientific community.

“It’s game over for the plaintiffs if they can’t get over this hurdle,” said David Levine, an expert in federal court procedure at the University of California, Hastings College of the Law.

The U.S. Environmental Protection Agency says glyphosate is safe for humans when used in accordance with label directions. A draft report by the agency last year concluded the herbicide is not likely to be carcinogenic to humans. The report noted science reviews by numerous other countries as well as a 2017 National Institute of Health survey had reached the same conclusion.

“There are more than 800 published studies — scientific, medical and peer-reviewed — which demonstrate that glyphosate is safe and there is no association whatsoever with any form of cancer,” said Scott Partridge, vice president of strategy at Monsanto.

A federal judge in Sacramento last week blocked California from requiring that Roundup carry a label stating that it is known to cause cancer, saying the warning is misleading because almost all regulators have concluded there is no evidence glyphosate is a carcinogen.

Monsanto developed glyphosate in the 1970s, and the weed killer is now sold in more than 160 countries. Farmers in California use it on more than 200 types of crops. Homeowners use it on their lawns and gardens.

St. Louis-based Monsanto also sells seeds that can tolerate being sprayed with glyphosate as the surrounding weeds die, ensuring another stream of business that has helped it dominate the market for genetically modified crops.

But the herbicide came under increasing scrutiny after the International Agency for Research on Cancer, based in Lyon, France — part of the World Health Organization — classified it as a “probable human carcinogen” in 2015. A flurry of lawsuits against Monsanto in federal and states courts followed, and California added glyphosate to its list of chemicals known to cause cancer. Monsanto has attacked the international research agency’s opinion as an outlier.

Christine Sheppard, among those suing Monsanto, said she sprayed Roundup for years to control weeds on her Hawaii coffee farm. In 2003, she was diagnosed with non-Hodgkin’s lymphoma and given six months to live. Now 68, she is in remission but experiences severe pain in her hands and legs from her cancer treatment and has a weak immune system. She believes Roundup is to blame.

“The thing that really gets to me right now is when I walk into Home Depot and places like that and see Roundup still for sale, still advertised as the best thing people can use,” said Sheppard, who now lives near San Diego.