SGCG and fibrodysplasia ossificans progressiva: Fibrodysplasia ossificans progressiva (FOP) is a very rare, severe genetic disorder triggered by a gain-of-function mutation in the ACVR1 gene that codes for the type I bone morphogenetic protein (BMP) receptor ACVR1 (activin A receptor—type 1), also known as ALK2 (activin receptor-like kinase-2).