ACVR1 and fibrodysplasia ossificans progressiva: Its worldwide prevalence is estimated to be 1 case per 2 million individuals.[1] Recently, it was shown that heterozygous activating mutations in the activin A receptor, type I/activin-like kinase 2 (ACVR1/ALK2) are primarily responsible for this disease.[2] Ossification commonly begins in the first decade of life, ranging from 6 months to 13 years.[3] The main clinical features of FOP are malformations of the great toes and painful soft-tissue swelling that eventually progresses to heterotopic ossification.