SAN FRANCISCO — Hundreds of lawsuits alleging Roundup weed killer caused cancer cleared a big hurdle Tuesday when a U.S. judge ruled that cancer victims and their families could present expert testimony linking the herbicide to non-Hodgkin’s lymphoma.

U.S. District Judge Vince Chhabria said evidence that the active ingredient in Roundup – glyphosate – can cause the disease seemed “rather weak.” Still, the opinions of three experts linking glyphosate and non-Hodgkin’s lymphoma were not “junk science” that should be excluded from a trial, the judge ruled.

The lawsuits say agrochemical giant Monsanto, which makes Roundup, long knew about the cancer risk but failed to warn people. The ruling allows the claims to move forward, although the judge warned it could be a “daunting challenge” to convince him to allow a jury to hear testimony that glyphosate was responsible for individual cancer diagnoses.

Many government regulators have rejected a link between cancer and glyphosate. Monsanto has vehemently denied such a connection, saying hundreds of studies have established that the chemical is safe.

The company is facing hundreds of lawsuits that claim otherwise. Chhabria is presiding over more than 400 of them.