Material Event (Form 8-K)

On July 9, 2025, Ultragenyx Pharmaceutical Inc. (the "Company") announced that the randomized, placebo-controlled Phase 3 portion of the Orbitstudy evaluating UX143 (setrusumab) in pediatric and young adult patients with osteogenesis imperfecta ("OI") is progressing toward a final analysis consistent with the original plan, around the end of the year.

The Data Monitoring Committee (the "DMC") met and informed the Company that UX143 demonstrates an acceptable safety profile and the Company should continue the study to the final analysis.

Consistent with the statistical analysis plan, data from the UX143 Cosmicstudy were not analyzed at this interim timepoint. Study conduct is going well and safety in this younger patient population is consistent with the safety profile in the other studies.

Patients will continue dosing in the ongoing Phase 3 Orbitand Cosmic clinical studies with the final analyses to be conducted after patients have been on therapy for at least 18 months. The threshold for the Phase 3 Orbit final analysis is p<0.04 and for the Phase 3 Cosmicfinal analysis is p<0.05.