Whereas WT ACVR1 forms nonsignaling complexes with activin A and the corresponding type II receptors (9), FOP-mutant ACVR1 is activated by activin A. This neofunction is essential for HO in FOP, as inhibition of activin A using mAbs ameliorates the initiation and progression of heterotopic bone lesions in FOP mice (8, 10, 13). Here, ACVR1 is linked to fibrodysplasia ossificans progressiva.