• The requirement that targeted radiographic assessments to diagnose and monitor skeletal abnormalities identified during routine standard of care assessments be reported as an unscheduled visit in the eCRF and evaluated centrally was removed as there was no requirement to collect these data as part of the study. Any skeletal abnormalities identified during the study were reported as AEs or SAEs, as appropriate. • A new Clinical Trial Manager was appointed.

• Serum creatinine and serum calcium were collected at the same timepoints as for spot urine in order to be able to calculate the TmP/GFR and the creatinine/calcium ratio. No additional blood sampling was required. • Following recommendation by the DSMB and because formula milk contained higher levels of phosphate, the nutritional status (i.e., breast fed, bottle fed with formula milk, weaned, dietary changes, etc) of the subject was recorded at intervals throughout the study. • The procedure for increasing a patient’s dose was clarified. • The Baseline renal ultrasound was moved to Screening as it formed part of the inclusion criteria. • In order to allow pharmacies sufficient time to prepare the first dose an additional weight measurement was taken at Screening. In addition, the study schedule of events table was corrected to include weight measurements on Day 14 and Week 6. • In cases where the genetic testing for PHEX mutation was required, the testing was performed during Screening or within the first 20 weeks on the study, rather than within the first 24 weeks on the study. • The serum phosphate analysis used to determine the patient’s hypophosphatemia status for the inclusion criteria was conducted by a local laboratory. • The requirement to enter AE information in the eCRF within 24 hours was removed. • It was clarified that a Laboratory Manual detailing blood sample collection, preparation, labeling, and storage was only provided for the Central Laboratory. • All subjects were assessed for the occurrence of AEs from the time following the first dose of investigational product until 56 days after the last dose or until Week 76 (whichever was longer). • Details on the reporting of any COVID-19 infections were included.

• The planned treatment duration was updated from at least 64 weeks to up to 48 weeks since the safety and efficacy of burosumab had previously been demonstrated in other clinical trials included in the marketing authorization. A 48 week study did not limit access of the patient to treatment as the product was indicated for patients from 12 months of age, and given that the study provided for a 30-day Screening period followed by 48 weeks of treatment, this ensured that any patient completing the study were more than 12 months old at study completion. Consequently, the frequency and timing of certain assessments were amended. • The study design had been revised to remove Cohort 4 and the Expansion Part (Part 2). There was more flexibility for dosing of subjects <6 months of age in Cohort 3, allowing the starting dose to be increased to 0.8 mg/kg if deemed safe and appropriate. • The requirement to offer subjects participation in a 96-week follow-up extension study was removed, as data on effects of long-term treatment were available from 3 studies in pediatric XLH subjects aged 1 to 12 years (UX023 CL201, UX023 CL205, and UX023 CL301); therefore, there was no need for a further extension study after completion of BUR CL207. Subjects were offered long term follow-up in the Kyowa Kirin Registry upon consent by their legally authorized representative in territories where this was available at the time the subject completed 48 weeks of treatment. • The exploratory endpoint to measure time to appearance of radiological abnormalities due to XLH was removed as it could not be evaluated with only 2 X rays. • Day 3 and Day 11 visits were removed to reduce the burden of visits for families and the site during the initial 2 weeks and PK timepoints had been updated to match PD timepoints. • The Burosumab Dosing Recommendations tables were removed for simplification.

• The minimum number of evaluable subjects to be enrolled into the study was updated to at least 14 in line with the PIP commitment. • Exclusion criterion 3 was updated to remove the wording “and estimated GFR (eGFR, calculated using the Bedside Schwartz equation) below the age-adjusted normal range”, since this equation was not developed and validated in infants below 12 months of age and is thus not applicable to the patient population to be enrolled in this study. • A timepoint was added at Week 26 for 1,25(OH)2D as it had previously not been included in error. • Subgroup analysis performed for age group was removed as analyses would only be performed by cohort. • Wording was updated regarding the marketing authorization and clinical studies completion statuses. • Wording was added to the Study Schedule of Events to clarify that if spot urine could not be collected this would not be a protocol deviation. • The requirement for the calculation of percentiles for age-adjusted normal ranges to be provided for each Investigational Site before enrolling the first subject was deleted. • The Sponsor Medical Monitor contact details were updated.