Inclusion Criteria:

• Have a clinical diagnosis of TIO/ENS based on evidence of excessive FGF23 that is not amenable to cure by surgical excision of the offending tumor/lesion (documented by investigator).
• Be ≥18 years of age.
• Have a fasting serum phosphorus level <2.5 mg/dL
• Have an FGF23 level ≥ 100 pg/mL by Kainos assay.
• Have a ratio of renal tubular maximum reabsorption rate of phosphate to glomerular filtration rate (TmP/GFR) <2.5 mg/dL.
• Have an estimated glomerular filtration rate (eGFR) ≥60 mL/min (using Cockcroft-Gault formula). Subjects with eGFR ≥30 but <60 mL/min will be considered eligible as long as in the opinion of the investigator the decline in renal function is not related to nephrocalcinosis.
• Have a corrected serum calcium level <10.8 mg/dL.
• Females of child-bearing potential must have a negative urine pregnancy test at Screening and Baseline and be willing to have additional pregnancy tests during the study. Females considered not to be of childbearing potential include those who have not experienced menarche, are post-menopausal (defined as having no menses for at least 12 months without an alternative medical cause) or are permanently sterile due to total hysterectomy, bilateral salpingectomy, or bilateral oophorectomy.
• Participants of child‐bearing potential or fertile males with partners of child-bearing potential who are sexually active must consent to use a highly effective method of contraception as determined by the site Investigator from the period following the signing of the informed consent through the final Safety Follow-up TC.
• Be willing to provide access to prior medical records to determine eligibility including imaging, biochemical, and diagnostic, medical, and surgical history data.
• Provide written informed consent after the nature of the study has been explained, and prior to any research-related procedures.
• Be willing and able to complete all aspects of the study, adhere to the study visit schedule and comply with the assessments (in the opinion of the investigator).

Exclusion Criteria:

• Have a prior diagnosis of human immunodeficiency virus (HIV), hepatitis B and/or hepatitis C.
• Have a history of recurrent infection, a predisposition to infection, or a known immunodeficiency.
• Are pregnant or breastfeeding at Screening or are planning to become pregnant (self or partner) at any time during the study.
• Have participated in an investigational drug or device trial within 30 days prior to Screening or are currently enrolled in another study of an investigational product or device.
• Have used a therapeutic monoclonal antibody, including KRN23, within 90 days prior to Screening or have a history of allergic or anaphylactic reactions to any mAb.
• Have or a have a history of any hypersensitivity to KRN23 excipients that, in the judgment of the investigator, places the subject at increased risk for adverse effects.
• Have used a pharmacologic vitamin D metabolite or its analog (e.g., calcitriol, doxercalciferol, and paricalcitol), phosphate, or aluminum hydroxide antacids (e.g., Maalox® and Mylanta®) within 2 weeks prior to Screening or during the study.
• Have used medication to suppress PTH (e.g., Sensipar®, cinacalcet, calcimimetics) within 2 months prior to Screening.
• Have a history of malignancy within 5 years of study entry with the exception of PMT-MCT (Phosphaturic mesenchymal tumors of the mixed connective tissue type) tumors or non-melanoma skin cancers such as basal cell skin cancer.
• Have donated blood or blood products within 60 days prior to Screening.
• Have a history of allergic reaction to or have shown adverse reactions to a tetracycline (e.g., tetracycline HCl and demeclocycline), benzodiazepines, fentanyl or lidocaine.
• Have any condition, which in the opinion of the investigator and sponsor, could present a concern for either subject safety or difficulty with data interpretation.