Importantly, both Runx2-deficient mice and chondrocyte-specific Runx2–transgenic mice display abnormal cartilage development8, 9, and Runx2-haploinsufficient mice show reduced type X collagen and MMP13 protein and mRNA levels accompanied by decreased cartilage degradation in osteoarthritis (OA) models16. The gene discussed is RUNX2; the disease is osteoarthritis.