Study to Assess the Safety, Tolerability, Pharmacokinetics and Efficacy of Burosumab in Patients Less Than 1 Year of Age

Inclusion Criteria: 1. Male or female pediatric subjects, aged \<12 months at burosumab treatment initiation. 2. Pediatric subjects with PHEX mutation or variant of uncertain significance in either the subject or a directly related family member with appropriate X-linked inheritance. 3. Presenting serum phosphate levels below the age-specific LLN at Screening. 4. A legally authorized representative has provided written informed consent prior to any research-related procedures. 5. A legally authorized representative must, in the opinion of the Investigator, be willing and able to complete all aspects of the study, adhere to the study visit schedule, and comply with the assessments required by the study protocol, including providing access to prior medical records for the collection of historical growth, biochemical, and radiographic data and disease history. Exclusion Criteria: 1. The pediatric subject's legally authorized representative is unwilling or unable to stop the subject's treatment with oral phosphate and/or pharmacologic vitamin D metabolite or analogue (e.g. calcitriol, alfacalcidol) for at least 1 week before planned treatment start and for the duration of the study. 2. Preterm pediatric patients (defined as born before 37 weeks of pregnancy) with a chronological age of \<6 months. Enrolment of preterm pediatric patients with a chronological age ≥6 months must be confirmed by the Study Medical Monitor before study entry. 3. Impairment of renal function measured as serum creatinine above the age-adjusted normal range and estimated GFR (calculated using the Bedside Schwartz equation) below the age-adjusted normal range. 4. Presence of nephrocalcinosis on renal ultrasound. 5. Hypocalcemia or hypercalcemia, defined as serum calcium levels outside the age-adjusted normal limits. 6. Presence of a concurrent disease or condition that would interfere with study participation or affect subject safety. 7. Predisposition to infection or known immunodeficiency. 8. Severe dermatological conditions over the available injection sites. 9. Use of any investigational product or investigational medical device within 30 days prior to Screening, or requirement for any investigational agent prior to completion of all scheduled study assessments. 10. Metabolic bone disease, nutritional rickets and/or osteopenia of other origin than XLH at Screening and/or Baseline. 11. Serum levels of 25-hydroxyvitamin D (25(OH)D) below the LLN that are clinically significant in the opinion of the Investigator. 12. Evidence of any hyperparathyroidism not associated with XLH as determined by the Investigator.