Shares of Sarepta Therapeutics Inc. have lost more than 26% this week, heading for their worst such performance in over a year, after the company disclosed the death of a Duchenne muscular dystrophy (DMD) patient treated with its Elevidys drug.

Sarepta Therapeutics said a patient taking its treatment for Duchenne muscular dystrophy has died from acute liver failure. The pharmaceutical firm said that "acute liver injury is a known possible side effect of Elevidys.

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.