Stock in Pfizer dropped on Tuesday after a young boy receiving experimental gene therapy for Duchenne muscular dystrophy died during the trial.

According to STAT News, Pfizer has yet to determine exactly what happened or how the boy died Pfizer has confirmed that the boy suffered cardiac arrest, and is pausing crossing over patients in its randomized phase 3 trial of the treatment in older boys, according to the report. The company will, for now, stop dosing patients who initially received a placebo with the gene therapy after a year.

Pfizer is expected to announce data from the study in the upcoming weeks.