Sarepta Therapeutics stock plummeted Monday after the company said a second patient receiving its gene therapy died from liver failure, raising serious questions on the drug’s outlook.Shares fell 40% to $21.

Sarepta’s shares crashed 41% in premarket trading Monday morning to $21.01 after the biotech reported a second death from acute liver failure, a known side effect of adeno-associated virus-based gene therapies.

Sarepta Therapeutics said a second patient had died after receiving its $3.2 million gene therapy to treat Duchenne muscular dystrophy.