The objectives of this observational study are to characterize XLH disease presentation and progression and to assess long-term effectiveness and safety of burosumab.

The XLH-DMP is a global, prospective, multicenter, longitudinal, long-term outcomes program for subjects on or off any treatment designed to characterize XLH disease presentation and progression, assess long-term safety and effectiveness of burosumab, as well as prospectively investigate longitudinal change over time across biomarker(s), clinical assessments, and patient/caregiver-reported outcome measures in a representative population. The XLH-DMP will collect demographic, biochemical, physiologic, disease severity, and progression data in patients taking burosumab and those not taking burosumab. In this DMP, patients will only have access to burosumab through authorized prescribed use. The Sponsor will not provide any treatments as part of the DMP.

Inclusion Criteria:

• Willing and able to provide informed consent or, in the case of patients under the age of 18 years (or 16 years, depending on the region), provide assent (if required) and informed consent by a legally authorized representative after the nature of the study has been explained, and prior to any research-related procedures.
• Clinical diagnosis of XLH based on family history, OR confirmed PHEX mutation, OR biochemical profile consistent with XLH.
• Willing and able to comply with the study visit schedule and study procedures.

Exclusion Criteria:

• Concurrent enrollment in an Ultragenyx-sponsored clinical trial is NOT permitted.
• Serious medical or psychiatric comorbidity.
• Less than one year of life expectancy.

Approximately 800 eligible adult and pediatric patients with XLH will be enrolled globally, with a minimum of 200 pediatric patients. Patients can enter the XLH-DMP regardless of how their XLH is being treated. Patients on Crysvita (burosumab) via prescription may begin taking Crysvita, per standard of care before or after enrolling in the XLH DMP.