ACVR1 and fibrodysplasia ossificans progressiva: By contrast, fibrodysplasia ossificans progressiva (FOP) is a rare monogenic condition in which a gain of function germline mutation in ACVR1 leads to increased signalling through ALK2 in response to BMP ligands as well as neofunction in response to Activin A and the consequent formation of heterotopic bone in muscle and connective tissue [11], [12].