RARE: Other company developments

Item 8.01 Other Events. On September 17, 2026, Ultragenyx Pharmaceutical Inc. (“Ultragenyx” or the “Company”) announced that the U.S. Food and Drug Administration (the “FDA”) granted standard full approval of FAYUVI™ (rebisufligene etisparvovec-hopf), also known as UX111, for the treatment of pediatric patients with mucopolysaccharidosis type IIIA (MPS IIIA, Sanfilippo syndrome Type A). FAYUVI is the first-ever FDA-approved treatment for Sanfilippo syndrome Type A, a progressive and fatal neurodegenerative disease, and the second gene therapy approval for Ultragenyx. The Company received a Priority Review Voucher upon this approval. SIGNATURES Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized. Ultragenyx Pharmaceutical Inc. Date: September 17, 2026 By: /s/ Howard Horn Howard Horn Executive Vice President, Chief Financial Officer, Corporate Strategy