ACVR1 and osteoarthritis: Alternatively, these mice were i.v. injected with a single dose of rAAV9.amiR-RH6.ACVR1opt and 12 weeks later, control-treated Acvr1(R206H)Fl;Cre-ERT2 mice progressively developed osteochondromas in the tibia, osteoarthritis in the knees, and spontaneous HO at multiple anatomical locations, including the cervical spine, hips, and knees—skeletal features that are commonly seen in individuals with FOP53.