ACVR1 and fibrodysplasia ossificans progressiva: The resulting mouse model of FOP, Acvr1[R206H]FlEx/+; GT(ROSA26)SorCreERT2/+, is genotypically rendered FOP by systemic treatment with tamoxifen to activate CreERT2 and convert the Acvr1[R206H]FlEx allele to Acvr1R206H.