Fibrodysplasia Ossificans Progressiva

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)

The participants in this registry study will have fibrodysplasia ossificans progressiva (FOP). FOP is an ultra-rare, severely disabling disease characterized by new bone formation in areas of the body where bone is not normally present (heterotopic ossification (HO)). HO is often preceded by painful, recurrent episodes of soft tissue swelling (flare-ups). This registry study will take place in countries where the treatment, known as palovarotene, has been approved for use. Participants will either be treated with palovarotene (i.e already be receiving palovarotene as prescribed by their treating physician according to locally approved product information) or untreated with palovarotene. The main aim of this registry study will be to collect and assess real-world safety data on children and adult participants with FOP treated with palovarotene. This registry study will also describe the effectiveness of palovarotene in exposed participants, including the effect on everyday activities and physical performance. In addition, this registry study aims to descriptively compare key safety outcomes (i.e. flare-up episodes, growth outcomes, and bone fractures) between participants exposed and unexposed to palovarotene.