ACVR1 and fibrodysplasia ossificans progressiva: In order to determine whether the ability of Activin A to activate FOPcausing variants of ACVR1 bears any relevance to the pathophysiology of FOP, and particularly HO, we tested whether Activin A can induce the formation of heterotopic bone in Acvr1[R206H]/+ mice, using a classic osteogenic assay wherein the ligand is adsorbed into collagen sponges, implanted intramuscularly, and the formation of bone within and around the implants is monitored [16].