ACVR1 and fibrodysplasia ossificans progressiva: In contrast, dominantly inherited gain-of-function mutations in the BMP receptor ACVR1/ALK2, which render the receptor constitutively active even in the absence of BMPs (8) and that are further activatable by Activin, a member of the other TGFβ superfamily subgroup (9), cause Fibrodysplasia Ossificans Progressiva (FOP) (9–11).