FOP

This study is researching an experimental drug called garetosmab, referred to as "study drug". The study is focused on children and adolescent participants with FOP. For United States: Ages 4 to less than 18 years; for non-US sites only: Ages 2 to less than 18 years. The aim of the study is to see how safe, tolerable, and effective the study drug is. The study is looking at several other research questions, including: * What side effects may happen from taking the study drug * How much study drug is in the blood at different times * Whether the body makes antibodies against the study drug (which could make the study drug less effective or could lead to side effects)