Citron Research's Andrew Left, who gained notoriety for successful bets against companies such as Valeant Pharmaceuticals, posted a bearish report on cancer testing company Exact Sciences.
Shares of Exact Sciences closed down nearly 4 percent Monday after the short seller released his report.
"Exact Sciences pushes a cancer test (Cologuard) to the public, inferior by its own admission, and loses money doing it," Left wrote in the note. "Anyone who follows the landscape of diagnostic cancer testing knows that everything is moving towards blood-based DNA testing. Any doctor in the field will tell you it is just a matter of time."
The short seller also cited the low number of test orders per doctor, according to his analysis, and the potential for reduced reimbursement from Medicare in the future.
Exact Sciences has "an inferior test that they're trying to compensate by running commercials for people who are afraid to get a colonoscopy," Left said Monday on CNBC's "Halftime Report." "They are losing money selling an inferior product."
However, Exact Sciences CEO Kevin Conroy fired back on CNBC directly at the short seller.
Left "is dead wrong," Conroy said. "Blood-based tests is very unlikely to ever displace any other tests. You want to detect stage one cancers and you just can't do that reliably from blood."
— With reporting by CNBC's Meg Tirrell.