Fibrodysplasia ossificans progressiva (FOP), a rare genetic disorder associated with progressive soft tissue ossification, is caused by a conserved (R206H) substitution in the BMP type I receptor, ACVR1, allowing it to be abnormally stimulated by activin A, which normally suppresses BMP stimulation of the wild-type receptor33. Here, ACVR1 is linked to fibrodysplasia ossificans progressiva.