Treatment-emergent adverse events (TEAEs) are adverse events not present at baseline or represent the exacerbation of a pre-existing condition during the on-treatment period. A serious TEAE was defined as any untoward medical occurrence that resulted in any of following outcomes not present at baseline or represent the exacerbation of a pre-existing condition during the on-treatment period: death, life-threatening, required initial/prolonged in-participant hospitalization, persistent/significant disability/incapacity, congenital anomaly/birth defect/considered as medically important event. Number of participants with TEAEs and Serious TEAEs are reported.

Severity of TEAEs were graded as follows: Mild: Does not interfere in a significant manner with the participant's normal functioning level. It may be an annoyance. Prescription drugs are not ordinarily needed for relief of symptoms but may be given because of personality of the participants. Moderate: Produces some impairment of functioning but is not hazardous to health. It was uncomfortable or an embarrassment. Treatment for symptom may be needed. Severe: Produces significant impairment of functioning or incapacitation and was a definite hazard to the participant's health. Treatment for symptom may be given and/or participants hospitalized. Number of participants with TEAEs by severity is reported.

18\^F-NaF PET is used to assess lesion and disease activity. Time-weighted average (standardized area under the curve \[AUC\]) of the percent change from baseline in total lesion activity by 18\^F-NaF PET up to Week 28 in AHO analysis set is reported.

CT is a diagnostic imaging test used to create detailed images of internal organs, bones, soft tissue, and blood vessels. CT scan acquired contemporaneously to the PET scan. Percent change from baseline in the total volume of HO lesions as assessed by CT during Period 1 at Week 28 is reported.

CT is a diagnostic imaging test used to create detailed images of internal organs, bones, soft tissue, and blood vessels. CT scan acquired contemporaneously to the PET scan. HO detectable by CT that developed after baseline are referred to as "new HO lesions." Number of new HO lesions as assessed by CT at Week 56 relative to Week 28 scan is reported.

18\^F-NaF PET is used to assess lesion and disease activity. Time-weighted average (Standardized AUC) of the percent change from baseline in total lesion activity as assessed by 18\^F-NaF PET in Active HO Classic ACVR1 Mutation (AHOC) analysis set up to Week 28 is reported.

CT is a diagnostic imaging test used to create detailed images of internal organs, bones, soft tissue, and blood vessels. CT scan acquired contemporaneously to the PET scan. Percent change from baseline in the total volume of HO lesions was assessed by CT at Week 28 in AHOC analysis set is reported.