ACVR1 and fibrodysplasia ossificans progressiva: Although it appears that human ACVR1[R206H] is less active than its mouse counterpart, these results strongly caution against the use of anti-ACVR1 antibodies as therapeutic agents to block HO in FOP, because they clearly induce more HO than that observed when FOP[S330P] mice are dosed with a control antibody (akin to placebo) and they may even induce anemia.