ACVR1 and fibrodysplasia ossificans progressiva: To test the effect of saracatinib in a more faithful model of FOP disease, we employed a more recently described conditional Acvr1R206H-knockin mouse, in which the Acvr1R206H mutant allele (Acvr1[R206H]FlEx/+]) is conditionally expressed within the mouse Acvr1 locus following Cre-loxP–mediated recombination (13).