FOP is caused by activating mutations in the ACVR1 gene that lead to increased canonical signaling by the BMP pathway4 and neo-ligand activity by Activin A.6,7 The known mutations in FOP are primarily located in the glycine-serine (GS) region of ALK2, with the most common being the single-point ACVR1 mutation c. The gene discussed is ACVR1; the disease is fibrodysplasia ossificans progressiva.