Material Event (Form 8-K)

Item 8.01 Other Events.

On December 30, 2025, Ultragenyx Pharmaceutical Inc. (the "Company") issued a press release announcing that that it has completed the rolling submission of its Biologics License Application ("BLA") to the U.S. Food and Drug Administration (the "FDA") seeking approval for DTX401 AAV gene therapy (pariglasgene brecaparvovec) as a treatment for Glycogen Storage Disease Type Ia ("GSDIa"). The BLA for DTX401 is based on data from a clinical development program that includes 52 treated patients and up to six years of follow-up. Previously announced data from the randomized, double-blind, placebo-controlled Phase 3 GlucoGene study demonstrate that patients treated with DTX401 experienced significant and clinically meaningful reductions in both the quantity and frequency of daily cornstarch intake while maintaining low levels of hypoglycemia, improved levels of euglycemia, and improved fasting tolerance. These clinical benefits translated to meaningful improvements in patient-reported quality of life, as measured by the Patient Global Impression of Change scale. DTX401 was well tolerated with an acceptable safety profile.

The Company was previously granted rolling review and submitted the non-clinical and clinical modules to the FDA in August 2025 and has now completed the package through submission of the chemistry, manufacturing and controls module.