Upon Cre-mediated recombination, the Acvr1[R206H]FlEx allele is converted to Acvr1R206H, and the corresponding mice (Acvr1R206H/+; GT(ROSA26)SorCreERT2/+, here on referred to as FOP mice) develop HO in response to injury [29,37], mimicking what is observed in FOP patients. The gene discussed is ACVR1; the disease is fibrodysplasia ossificans progressiva.