Clinical-stage biopharmaceutical company Regenxbio (RGNX) saw its shares plunge 13.3% early Tuesday after the U.S. Food and Drug Administration (FDA) issued a complete response letter for its one-time gene therapy RGX-121, developed for Hunter syndrome. The FDA raised concerns regarding surrogate endpoints and the comparability of control groups.

Hunter syndrome is a form of peripheral facial paralysis caused by the reactivation of the varicella-zoster virus.

Regenxbio stated that it plans to request a Type A meeting with the agency and may resubmit the application.