ACVR1 and fibrodysplasia ossificans progressiva: Likewise, elevated levels of Activin A were also detected in bone marrow-derived stromal cells (BMSCs) isolated from FOP mice (Acvr1R206H;Prx1) relative to WT mice (Acvr1+/+;Prx1,Figure 1B), indicating that heterotopic bone-forming, chondrogenic/osteogenic cells are also Activin A-producing cell populations in FOP.