Regenxbio said on Monday the U.S. FDA has placed its experimental gene therapy for a ​rare inherited disorder on clinical hold after spinal scans found ‌abnormalities in five study participants, sending its shares down more than 24% in premarket trading.

The therapy, RGX-121, was being developed as a one-time treatment for Hunter Syndrome, also ​known as MPS II, which can cause progressive damage to ​the brain and other organs and tissues.

The spinal scans identified ⁠a small lump or fluid-filled mass in the patients who received the ​treatment, RGX-121, about three to six years ago, the biotech said.

The setback ​adds to Regenxbio’s regulatory troubles after the U.S. Food and Drug Administration declined to approve RGX-121 earlier this year over concerns about the trial design and supporting evidence.

RGX-121 was ​also subject to the health regulator’s previous clinical hold on the company’s two ​gene therapy programs.

The therapy uses a harmless virus known as AAV9 vector to deliver ‌the ⁠gene needed to produce the missing enzyme in patients with Hunter syndrome.

Current treatment options include Takeda Pharmaceutical’s Elaprase, a weekly infusion that treats the physical effects of Hunter syndrome, and Denali Therapeutics’ Avlayah, approved in March, to ​treat neurological symptoms in ​certain children.

Regenxbio ⁠said the five patients remained asymptomatic and were clinically stable or showed improvements in cognitive and behavioral assessments. Trial ​investigators deemed the findings to be non-serious and radiologists ​believed ⁠they were likely benign.

The company said it does not expect to resubmit its application for the treatment’s approval in the near term.

It, along with partner ⁠NS Pharma, ​will review additional imaging and longer-term follow-up ​data and await the FDA’s full clinical hold letter before deciding the next steps for ​RGX-121.