Claims that the active ingredient in the widely used weed killer Roundup can cause cancer have been evaluated by international agencies, U.S. and foreign regulators and the product's manufacturer — agribusiness giant Monsanto.

Now, a federal judge in San Francisco is conducting his own review during an unusual set of court hearings that began Monday. It has big stakes for Monsanto and hundreds of farmers and others who have sued the company.

U.S. District Judge Vince Chhabria will spend a week hearing from experts to help decide whether there is valid scientific evidence to support the lawsuits' claim that exposure to Roundup can cause non-Hodgkin's lymphoma.

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.