We developed a conditional knockin mouse model of FOP in which expression of Acvr1R206H is Cre-dependent and under regulatory control of the endogenous Acvr1 locus (Acvr1tnR206H; Fig. 1a), enabling cell-specific and temporal control of Acvr1R206H expression. Here, ACVR1 is linked to fibrodysplasia ossificans progressiva.