This is an Observational Pre-post Study to Observe if the Off Label Use of Anti-IL1 Therapies, Such as Anakinra or Canakinumab, Can Block ACVR1-induced Flare Activity and Heterotopic Ossification in FOP

Eligibility Criteria

• * Patients with a clinical presentation consistent with FOP and a genetic diagnosis of classical FOP (ACVR1R206H variant) (2), male or female aged 6-17 years old.
• * Patients with unusually severe FOP disease activity. This will be determined by FOP flare frequency of \>6 flares per year, which is 3 times higher than the reported average in prior FOP studies ; or by a persistent flare that has failed to resolve after 3 months 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.
• * 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