ACVR1 and fibrodysplasia ossificans progressiva: ACVR1 gene (ALK2 protein; a bone morphogenetic protein (BMP) type I receptor) is the first identified human metamorphogene; its common missense mutation c.617G > A that substitutes arginine by histidine (p.Arg206His) causes skeletal metamorphosis in all classically affected FOP cases worldwide.