Call it the case of the mysterious mouse tumor.

It’s been 34 years since Monsanto Co. presented U.S. regulators with a seemingly routine study analyzing the effects the company’s best-selling herbicide might have on rodents. Now, that study is once again under the microscope, emerging as a potentially pivotal piece of evidence in litigation brought by hundreds of people who claim Monsanto’s weed killer gave them cancer.

This week tissue slides from long-dead mice in that long-ago research study are being scrutinized by fresh eyes as an expert pathologist employed by lawyers for cancer victims looks for evidence the lawyers hope will help prove a cover-up of the dangers of the weed killer called glyphosate.

Glyphosate, which is the active ingredient in Monsanto’s branded Roundup products, is the most widely used herbicide in the world, and is applied broadly in the production of more than 100 food crops, including wheat, corn and soy, as well as on residential lawns, golf courses and school yards.

Residues have been detected in food and human urine, and many scientists around the world have warned that exposure through diet as well as through application can potentially lead to health problems. The World Health Organization’s International Agency for Research on Cancer (IARC) declared glyphosate a probable human carcinogen in 2015 based on a review of scientific literature, triggering the wave of lawsuits against Monsanto, and pushing California regulators to announce they would add glyphosate to a list of known carcinogens.

What the expert finds, or doesn’t find, is expected to be key evidence in hearings slated for the week of Dec. 11 in dozens of consolidated cases being overseen by a federal judge in San Francisco.