Most cases of FOP are caused by a single amino acid substitution, R206H, in the type I BMP/TGFβ cell surface receptor ACVR1 (also known as ALK2), which over-activates signaling through phospho-Smad1/5 (pSmad1/5) (Shore et al., 2006; Shen et al., 2009; van Dinther et al., 2010; Fukuda et al., 2009). Here, ACVR1 is linked to fibrodysplasia ossificans progressiva.