One genetic condition that leads to massive and progressive HO is fibrodysplasia ossificans progressiva (FOP), a rare debilitating disease resulting from mutations in the Activin A Receptor Type 1 (ACVR1)/Activin-like Kinase 2 (ALK2) gene encoding for a type 1 membrane receptor for bone morphogenetic proteins (BMPs) [2]. This evidence concerns the gene CLN5 and fibrodysplasia ossificans progressiva.