Patient description: The patient is a 53-year-old man complaining of frequent headaches, generalized bone pain and difficulty chewing that started 6 years ago and is getting worse. Examination shows bilateral swellings around the molars. The swellings have increased since his last examination. Several extraoral lesions of the head and face are detected. The swellings are non-tender and attached to the underlying bone. Further evaluation shows increased uptake of radioactive substance as well as an increase in urinary pyridinoline. The serum alkaline phosphatase is 300 IU/L (the normal range is 44- 147 IU/L). The patient's sister had the same problems. She was diagnosed with Paget's disease of bone when she was 52 years old. The diagnosis of Paget's Disease of Bone is confirmed and Bisphosphonate will be started as first-line therapy.

Clinical trial eligibility criteria: eligible ages (years): 18.0-999.0,  X-linked Hypophosphatemia Male or female, aged ≥ 18 years, inclusive Diagnosis of X-linked Hypophosphatemia supported by classic clinical features of adult XLH (e.g. short statue or bowed legs, clinical symptoms as judged by the investigator) and at least one of the following at Screening visit documented PHEX mutation in either the patient, or in a directly related family member with appropriate X-linked inheritance Increased serum levels of c-term FGF23 or iFGF23 Biochemical findings consistent with XLH at Screening visit following overnight fasting Serum phosphorus level or TmP/GFR below lab specific lower limit of normal (LLN) Estimated glomerular filtration rate (eGFR) ≥ 60 mL/min (using the Chronic Kidney Disease Epidemiology Collaboration equitation) or eGFR of 30 up to 60 mL/min at Screening visit with confirmation that the renal insufficiency is not due to nephrocalcinosis Subjects who provide written informed consent after the nature of the study has been explained, and prior to any research-related procedures  Hypocalcemia or hypercalcemia, defined as serum calcium levels outside the age-adjusted normal limits and deemed as clinically significant in the opinion of the investigator Vitamin D deficiency (25OH D3 < 20ng/ml); if Vitamin D is low at screening, substitution is allowed and recompensation has to be confirmed before treatment start by normalized levels of Vitamin D (25OH D3 ≥ 20ng/ml) Serum intact parathyroid hormone (iPTH) >2.5-fold the upper limit of normal (ULN) Severe renal insufficiency with a Glomerular filtration rate (eGFR) <30 at screening Treatment with oral phosphate and / or active vitamin D analogues in addition to Burosumab treatment. (In order to ensure appropriate patient care and preclude any harm due to deficient supply, required supplementation with oral phosphate salts and/or active vitamin D analogues at screening can be continued during the run-in phase but has to be stopped before Baseline and Initiation of treatment with Burosumab.) Treatment with bisphosphates or Denosumab within the last 6 months Treatment with Teriparatide within the last 3 months Intake of calcimimetics within 30 days before screening Patients with known hypersensitivity to Burosumab and the active substances of any of the excipients of Burosumab Presence of a concurrent disease or condition that would interfere with study participation or affect safety in the opinion of the investigator