ACVR1 and fibrodysplasia ossificans progressiva: Fibrodysplasia ossificans progressiva (FOP; OMIM#135100) is an ultra-rare (1:800,000–1:3,000,000 prevalence worldwide) inherited condition that arises from missense mutations of the type I bone morphogenetic protein (BMP) receptor, activin A receptor type 1 (ACVR1)1–3.