On Wednesday, Regenxbio’s stock value dropped by nearly a fifth after the Food and Drug Administration (FDA) put a temporary hold on two of their experimental gene therapies. This action came after a case of brain cancer was found in a 5-year-old child who had received one of the treatments, RGX-111, four years earlier. The FDA decided to extend the hold to the second therapy, RGX-121, due to similarities between the two and shared risks. Regenxbio emphasized that there’s no proven link between RGX-111 and the child’s condition and that RGX-121 is a separate therapy with years of safety data. CEO Curran Simpson expressed surprise at the FDA’s decision to halt RGX-121 while investigating the single, inconclusive incident in RGX-111. The news is considered ‘confusing and unexpected’ by Leerink Partners analyst Mani Foroohar. The hold comes just days before the FDA’s deadline to approve or reject RGX-121 for treating Hunter syndrome, a rare condition affecting cells, tissues, organs, and the central nervous system. Foroohar had previously predicted an approval was ‘highly likely.’ Both RGX-121 and RGX-111 showed promise; in January 2025, Nippon Shinyaku agreed to pay $110 million upfront, with a possible $700 million more for rights to these therapies. RGX-111, designed to treat Hurler syndrome, is in earlier development. The child with cancer was part of an early-stage study and had no symptoms; the tumor was found during a routine brain scan. No similar issues have been seen in other patients treated with RGX-111 or RGX-121. While no causal link has been established, this setback highlights the potential risks in developing gene therapies, which offer great promise for patients with incurable and rare diseases but may also pose unknown dangers. Other companies like Capsida Biotherapeutics, Astellas Pharma, and Sarepta Therapeutics have reported deaths of patients who received their gene therapies. The FDA struggles to balance potential safety concerns with the need for innovative therapies. Regenxbio awaits further details from the FDA and believes the stock drop is overdone, considering their other therapies in development, including a treatment for Duchenne muscular dystrophy with trial results expected soon.