FOP is caused by mutations of the ACVR1 gene, encoding the ALK2 bone morphogenetic protein (BMP) type 1 receptor, and the consequent dysregulation of the BMP/Activin/TGF-ß family ligand signaling is a shared property of both genetic and acquired forms of HO (30, 31). Here, ACVR1 is linked to fibrodysplasia ossificans progressiva.