FOP is directly caused by mutations of the activin A receptor, type 1 (ACVR1) gene, which result in the synthesis of an abnormal activin receptor-like kinase 2 (ALK2) protein, leading to constitutive activity of BMP type I receptor and aberrant heterotopic ossification [37]. This evidence concerns the gene ACVR1 and fibrodysplasia ossificans progressiva.