ERRFI1 and arthropathy: Four weeks after surgery, we observed joint space narrowing in control knees from Mig-6−/−mice that was comparable to that previously reported in 1.5-month-old Mig-6−/−animals, but we did not observe joint space narrowing in WT control knees and therefore attribute this phenomenon to early-stage joint disease associated with loss of function in Mig-6−/−animals.