The FDA paused two Regenxbio gene therapy trials after the company reported a child participant developed a brain tumor, the company and STAT reported. The affected programs aim to replace broken genes in mucopolysaccharidosis type I and type II (Hurler and Hunter syndromes). Regenxbio had earlier sought approval for a Hunter syndrome candidate and has faced prior regulatory scrutiny; the agency’s pause follows the company’s disclosure and ongoing safety assessments. Regulators will require investigational teams to provide tumor evaluation data and causality analyses before trials can resume. The stoppage underscores the heightened safety bar for AAV and other in vivo gene therapies, particularly when treating pediatric, lifelong disorders where off‑target or oncogenic events carry severe consequences.