FAP and fibrodysplasia ossificans progressiva: As FOP patients are heterozygous for the Acvr1R206H mutation, and wild-type and ACVR1(R206H) receptors bind activins7–9 but exhibit distinct ligand-dependent signaling properties, we investigated the effect of genetic loss of wild-type Acvr1 to better understand the regulatory relationship between the wild-type and mutant receptors in FAP-directed HO.