GNAS and fibrodysplasia ossificans progressiva: There are two types of hereditary HO: fibrodysplasia ossificans progressiva (FOP) and progressive osseous heteroplasia (POH) [5]; FOP is due to a heterozygous mutation in the ACVR1 gene and involves endochondral ossification to form ectopic bone [21], while POH is caused by loss of function of the GNAS gene and involves ectopic bone formation through intramembranous ossification [3].