Finally, one of the best examples of this class of GSD is fibrodysplasia ossificans progressive (FOP), which results from heterozygous activating mutations in Activin receptor A, type I/Activin-like kinase 2 (ACVR1/ALK2), a bone morphogenetic protein receptor. This evidence concerns the gene ACVR1 and fibrodysplasia ossificans progressiva.