UX023-CL203 is a Phase 2b, open-label, long-term extension study. The study will be conducted in adults to assess the safety and pharmacodynamics of KRN23 administered via subcutaneous injections monthly (every 4 weeks) for a total of 68 weeks. The study will enroll approximately 25 adults with XLH who participated in Kyowa Hakko Kirin Pharma, Inc.'s (KHK's) study KRN23-INT-001 or KRN23-INT-002. Subjects will need to discontinue oral phosphate and vitamin D metabolite therapy prior to enrollment and throughout the duration of the study.