IL1 Inhibition in FOP

Inclusion Criteria:

* Patients with a clinical presentation consistent with FOP and a genetic diagnosis of classical FOP (ACVR1R206H variant) (2), male or female aged 6-30 years old.
* Patients with unusually severe FOP disease activity. This will be determined by FOP flare frequency of \>4 flares per year, which is 2 times higher than the reported average in prior FOP studies ; or by a persistent flare that has failed to resolve after 1 month of standard-of-care therapy.
* Patients whose primary medical team has decided that rescue therapy with an anti-IL1 medication should be initiated. Once the primary medical team has decided that anti-IL1 therapy should be pursued, the subject will be told about this clinical-observational study and enrolled in the pre-treatment phase while access to the anti-IL1 therapy is being obtained by the clinical management team.
* Ability to participate in all assessments, including blood draws, radiology assessments, and travel. Age 6 is chosen as the lower limit to avoid the need for anesthesia for whole body CT in younger subjects.
* No history of unexplained infections, known autoimmune disease, or contraindication to anti-IL1 therapy.
* Written informed consent (and assent when applicable) obtained from subject or subject's legal representative and ability for subject to comply with the requirements of the study.

Exclusion Criteria:

* Pregnant, breastfeeding, or unwilling to practice birth control during participation in the study.
* Presence of a condition or abnormality that in the opinion of the Investigator would compromise the safety of the patient or the quality of the data.
* Inability to travel to site for assessments
* Pre-existing autoimmune or autoinflammatory disease (aside from FOP)
* Inability to tolerate assessments (such as phlebotomy)
* Unexplained infections
* Current participation in an interventional trial, or study of a potentially disease modifying medication
* Inability to take medications as prescribed by managing physician