The primary objective of this study is to assess the safety and tolerability of KRN23 after a single intravenous (IV) and subcutaneous (SC) administration in XLH patients.

Inclusion Criteria:

1. 18 years or older
2. Clinical diagnosis of XLH
3. TmP/GFR is less than 2.0 mg/dL
4. GFR is 60 mL/min or above

Exclusion Criteria:

1. Have any sign of active infectious disease or have had an infection requiring treatment with antibiotics within three weeks prior to screening
2. History of known immunodeficiency
3. Lactating females, female patients who are pregnant or planning to become pregnant during the study
4. Use of a pharmacologic vitamin D metabolite or its analog, phosphate, calcimimetics, and ingestion of aluminum hydroxide antacids within 10 days prior to screening and dosing
5. Use of any supplement contained phosphate, calcium and/or vitamin D within 10 days prior to screening and dosing
6. Receipt of live (attenuated) vaccine within 3-months prior to screening
7. Have any condition which, in the opinion of the Investigator, could present a concern for either patient safety or difficulty with data interpretation