In support of this notion, in 6-month old Wnt9a∆Prx/− specimens ADAMTS5- and MMP13-positive cells were observed in the synovium (Supplementary Fig. 4b, c), and 9-month and older Wnt9a∆Prx/− mice develop osteoarthritis-like changes in their joints (C.H. and S.T. manuscript in preparation). The gene discussed is MMP13; the disease is osteoarthritis.