ACVR1 and fibrodysplasia ossificans progressiva: In 97% of cases, FOP arises from a de novo heterozygous mutation (c.617G>A; p.R206H) in the glycine and serine residue activation domain of activin A type I receptor (ACVR1)/activin-like kinase 2, a bone morphogenetic protein (BMP) type I receptor [2].