A patient has died while taking a closely watched gene therapy for muscular dystrophy. Sarepta Therapeutics announced the death Tuesday morning, sending the company's shares plummeting in morning trading.

Shares of Sarepta Therapeutics have struggled for the past year due to concerns about the commercial potential of its gene therapy and broader pipeline. Safety hadn’t been a major worry for investors—

The tragic death of the 16-year-old boy came as a result of acute liver failure, which is a known side effect of Elevidys and listed on its labelling and is thought to be linked to the adeno-associated virus (AAV) used to deliver the genetic material.