Material Event (Form 8-K)

On June 27, 2025, Ultragenyx Pharmaceutical Inc. (the "Company") issued a press release announcing that it has received Breakthrough Therapy Designation from the U.S. Food and Drug Administration (the "FDA") for GTX-102 (apazunersen) as a treatment for Angelman syndrome.

The FDA's decision is based on preliminary clinical evidence including positive data from the Phase 1/2 study in 74 patients (4-17 years of age) with a full maternal UBE3A gene deletion, that showed participants have made consistent developmental gains with rapid, sustained and continuing improvements across multiple symptom domains when treated for up to three years. Breakthrough Therapy Designation aims to expedite the development and review of drugs that are intended to treat serious or life-threatening diseases and whose preliminary clinical evidence indicates that the drug may demonstrate substantial improvement on one or more clinically significant endpoints over existing therapies.