Before switching to the post-marketing study:

Assess the efficacy and safety of KRN23 administered subcutaneously once every 4 or 2 weeks in adult or children with XLH

After switching to the post-marketing study:

To evaluate the safety and efficacy of KRN23, which was switched from the investigational product to the post-marketing investigational product, at the approved dose and dosing regimen in subjects who continued treatment

Inclusion Criteria:

1. Personally submitted voluntary written informed consent to participate in the study; For pediatric patients, personally submitted voluntary written informed consent by a legally authorized representative.

   If appropriate, written or verbal assent to participate in the study should be obtained from patients.

2. Patients meeting any of the followings;

   1. For adult XLH patients, completion the final observation at Week 96 in UX023-CL303 or UX023-CL304
   2. For pediatric patients, completion the final observation at Week 64 in UX023-CL301

3. For female patients; women of childbearing potential (except for females who have not reached menarche, permanently sterilized, postmenopausal [12 months with no menses without an alternative medical cause] or anatomically not of childbearing potential) with negative pregnancy test at pre-treatment assessment of Week 0

4. For female patient with childbearing potential, or male patients with reproductive capacity; willingness to use acceptable methods of contraception while participating in the study

5. Willingness and ability to cooperatively complete all study procedures, adhere to the visit schedule and follow the investigator's instructions, as considered by investigator or subinvestigator

Exclusion Criteria:

1. Use of oral phosphate for treating XLH, pharmacologic vitamin D metabolites or analogs, aluminum hydroxide antacids, systemic corticosteroids, acetazolamide, and thiazides within 7 days prior to scheduled initial administration of investigational drug
2. Planned or recommended orthopedic surgery (implantation or removal), including staples, 8 plates or osteotomy, during the study period
3. Blood or blood product transfusion within 60 days prior to scheduled initial administration of investigational drug
4. Use of growth hormone therapy within 12 months prior to scheduled initial administration of investigational drug
5. Use of medication to suppress the secretion of parathyroid hormone (e.g., cinacalcet) within 60 days prior to scheduled initial administration of investigational drug
6. Use of any investigational product (except for investigational product of the preceding study) or investigational medical device within 4 months prior to scheduled initial administration of investigational drug, or requirement for any investigational agent prior to completion of all scheduled study assessments
7. Use of a therapeutic monoclonal antibody other than KRN23 within 90 days prior to scheduled initial administration of investigational drug
8. History of being positive for HIV antibody, HBs antigen and/or HCV antibody
9. Anyone otherwise considered unsuitable for the study by the investigator or subinvestigator

At the time of switching to the post-marketing clinical study:

Subjects eligible for enrollment in the post-marketing clinical study must have met both of the following criteria:

1. Personally submitted voluntary written informed consent to participate in the postmarketing clinical study. For pediatric patients, personally submitted voluntary written informed consent by a legally authorized representative. If appropriate, written or verbal assent to participate in the post-marketing clinical study was to be obtained from subjects.
2. Switching to the post-marketing clinical study was necessary and appropriate for the subject from the viewpoint of efficacy and safety, as judged by the investigator or subinvestigator