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)

Key Inclusion Criteria:

1. 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

2. Must have a confirmation of FOP diagnosis, as described in the protocol

3. At the time of enrollment, participants must weight:

   1. Cohort 1 > 30 kg
   2. Cohort 2 > 30 kg
   3. Cohort 3 ≤ 30 kg

Key Exclusion Criteria:

1. Cumulative Analog Joint Involvement Scale (CAJIS) score > 19 at the time of screening
2. Participant has significant concomitant illness or history of significant illness, as described in the protocol
3. Previous history or diagnosis of cancer
4. Ongoing significant viral or bacterial illness, within 2 weeks of the first study drug administration
5. History of severe respiratory compromise requiring oxygen, respiratory support
6. Known history of cerebral vascular malformation
7. Participants with a history of severe, non-traumatic bleeding requiring transfusion or hospitalization for hemodynamic compromise
8. Participants with a known pre-existing medical history of a bleeding diathesis, as described in the protocol

NOTE: Other Protocol-defined Inclusion/Exclusion Criteria Apply

All Individual Patient Data (IPD) that underlie publicly available results will be considered for sharing.

When Regeneron has:

• received marketing authorization from major health authorities (e.g., FDA, European Medicines Agency (EMA), Pharmaceuticals and Medical Devices Agency (PMDA), etc.) for the product and indication or has globally discontinued development of the product for all indications on or after April 2020 and has no plans for future development
• made the study results publicly available (e.g., scientific publication, scientific conference, clinical trial registry)
• the legal authority to share the data, and
• ensured the ability to protect participant privacy

Qualified researchers can submit a proposal for access to individual patient or aggregate level data from a Regeneron-sponsored clinical trial through Vivli. Regeneron's Independent Research Request Evaluation Criteria can be found at: https://www.regeneron.com/sites/default/files/Regeneron-External-Data-Sharing-Policy-and-Independent-Research-Request-Evaluation-Criteria.pdf