Two independent studies demonstrated that an expression of IRX2 is commonly suppressed by DNA methylation of its promoter [43, 44], including its differential methylation noted in osteoarthritis and osteoporosis [45], two age-related diseases of the cartilage and the bone charactrized by changes in the levels of osteopontin secretion [46, 47]. The gene discussed is SPP1; the disease is osteoarthritis.