ACVR1 and fibrodysplasia ossificans progressiva: Fibrodysplasia ossificans progressiva (FOP) is an ultra‐rare (1:1,400,000 prevalence worldwide) and disabling inherited disorder that arises from missense mutations of the type I bone morphogenetic protein (BMP) receptor Activin A receptor type 1 (ACVR1).1, 2 To date, approximately 800 FOP cases have been documented.