Before switching to the post-marketing study:

To evaluate the efficacy and safety of KRN23 after its 144-week once every 4 weeks (Q4W) repeated SC administration to Japanese and Korean patients with TIO or ENS by a multicenter, open-label, intraindividual dose adjustment study.

After switching to the post-marketing study:

To evaluate the safety and efficacy of KRN23, which is switched from the investigational product to the post-marketing investigational product, at the approved dose and dosing regimen in subjects who continue treatment after the marketing approval of KRN23 in Japan.

Inclusion Criteria:

1. Aged ≥ 18 years
2. Diagnosis of Tumor-Induced Osteomalacia(TIO) or Epidermal Nevus Syndrome(ENS) and not amenable to receive surgical excision of the offending tumor/lesion
3. Serum phosphorus level < 2.5 mg/dL
4. Serum FGF23 level ≥ 100 pg/mL
5. Ratio of renal tubular maximum phosphate reabsorption rate to glomerular filtration rate< 2.5 mg/dL
6. Estimated glomerular filtration rate (eGFR) at screening ≥ 60 mL/min/1.73 m2, or eGFR ≥ 30 and < 60 mL/min/1.73 m2 with an evidence of no renal failure related to nephrocalcinosis
7. Corrected serum calcium level < 10.8 mg/dL
8. For female subjects of childbearing potential; negative urine pregnancy test and willingness to undergo additional pregnancy tests during the study
9. Willingness to use an acceptable method of contraception while participating in the study
10. Willingness to provide access to prior medical records to determine eligibility including data on imaging tests, blood chemistry, diagnosis, medication, and surgical history
11. Willingness and ability to cooperatively complete all study procedures, adhere to the visit schedule and follow the investigator's instructions, as considered by the investigator or subinvestigator

Exclusion Criteria:

1. Use of the following drugs within 14 days prior to screening: pharmacologic vitamin D metabolites or analogs, or drugs for treating TIO/ENS including oral phosphate, aluminum hydroxide antacids, acetazolamide, or thiazide diuretics
2. Medication to suppress parathyroid hormone (PTH) within 60 days prior to screening
3. Blood or blood product transfusion within 60 days prior to screening
4. Chemotherapy for TIO or other malignant tumors within 4 months prior to screening
5. History of being positive for human immunodeficiency virus antibody, hepatitis B antigen and/or hepatitis C virus antibody
6. Predisposition to infection, or history of recurrent infection or known immunodeficiency
7. Pregnant or breastfeeding at screening or intention to become pregnant during the study; for male subjects, the partner's intention to become pregnant during the study
8. Use of an investigational product or device within 4 months prior to screening, or planning to receive other investigational product before completing all assessments in this study
9. Use of therapeutic monoclonal antibodies including KRN23 within 90 days prior to screening
10. History of allergic or anaphylactic reactions to KRN23, any of the KRN23 ingredients, or any other monoclonal antibodies
11. Anyone otherwise considered unsuitable participation in the study by the investigator or subinvestigator

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

1. Voluntary written informed consent to participate in the post-marketing study (if aged < 20 years at the time of consent, written informed consent must be obtained from his or her legally acceptable representative as well)
2. Switching to the post-marketing study is necessary and appropriate for the subject from the viewpoint of efficacy and safety, as judged by the investigator or subinvestigator.