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.
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)
For USA participants, age criteria are 4 to < 18 years old, at the time of the administration of the first dose of study intervention. Non-USA participants age criteria are 2 to < 18 years old
Must have a confirmation of FOP diagnosis, as described in the protocol
Cohort 1 > 30 kg
Cohort 2 > 30 kg
Cumulative Analog Joint Involvement Scale (CAJIS) score > 19 at the time of screening
Participant has significant concomitant illness or history of significant illness, as described in the protocol
Previous history or diagnosis of cancer
Ongoing significant viral or bacterial illness, within 2 weeks of the first study drug administration