May 7 (Reuters) - A young boy died due to a cardiac

arrest in a mid-stage trial that tested Pfizer's ( PFE )

experimental gene therapy for a muscle-wasting disorder called

Duchenne muscular dystrophy (DMD), the drugmaker told Reuters on

Tuesday.

"A fatal serious adverse event was reported as cardiac

arrest for a participant in the Phase 2 DAYLIGHT study," a

company spokesperson told Reuters in an emailed response.

The trial tested boys 2 years to less than 4 years of age

with DMD, a genetic muscle wasting disorder in which most

patients lack the protein dystrophin which keeps muscles intact.

The disorder affects an estimated one-in-3,500 male births

worldwide.

"The patient received the investigational gene therapy,

fordadistrogene movaparvovec, in early 2023," as per a statement

from a community letter attributed to the drugmaker's DMD gene

therapy team and posted by a nonprofit advocacy group.

The company said, together with the independent external

data monitoring committee, it is in the process of reviewing the

data to understand the potential cause.