GNAS and progressive osseous heteroplasia: POH is a more recently characterized genetic form of progressive HO caused by heterozygous inactivating mutations in the GNAS1 gene.41 POH is an autosomal dominant disorder and can be a spontaneous/new mutation in the affected person or paternal inheritance of the mutant allele (OMIM:166350).41, 42 Ossification in POH has a predilection for the skin and subcutis and appears to be primarily intramembranous, although sporadic cartilage may also be found.