Ultragenyx Pharmaceutical Inc. has received a Complete Response Letter $(CRL)$ from the U.S. Food and Drug Administration (FDA) regarding its Biologics License Application $(BLA.AU)$ for UX111, an AAV gene therapy intended to treat Sanfilippo syndrome type A (MPS IIIA). The CRL cited specific chemistry, manufacturing, and controls $(CMC)$ related observations that need to be addressed. Although the CRL will delay the potential approval of UX111 to 2026, the company is actively working to resolve the issues and plans to resubmit the BLA. The FDA acknowledged that the clinical data are robust and supported by biomarker evidence, with no issues related to the clinical data package or inspections. Ultragenyx aims to address the CMC observations promptly and anticipates a resubmission followed by a 6-month review period.