Regulatory scrutiny tightened around AAV gene therapies this week as the FDA placed two Regenxbio studies on clinical hold after a patient in one trial developed cancer, and Ultragenyx formally resubmitted its NDA for UX111, an AAV program for Sanfilippo syndrome type A. The Regenxbio hold raises questions about long‑term malignancy surveillance in systemic AAV studies, while Ultragenyx’s resubmission follows the company’s prior rejection and indicates ongoing regulatory engagement. MedCity News and company disclosures note the Regenxbio hold follows an adverse oncology event that prompted FDA safety review; sponsors now must provide further data and monitoring plans. Ultragenyx’s resubmission signals continued effort to address FDA deficiencies for a rare‑disease gene therapy, underscoring how agencies are balancing unmet medical need against new safety signals in the AAV space. Both developments will shape near‑term trial designs, monitoring expectations, and approval pathways for in‑vivo gene therapies.