Early stages of sporadic breast cancer show a loss of the cell cycle checkpoint gene p16INK4a due to aberrant CpG promoter methylation, and approximately 80% of breast tumours show the decreased expression of another cell cycle inhibitor gene, p21/CIP1/WAF1, due to increased methylation of the p21/CIP1/WAF1 gene (Figure 11) [74,75,158,159,160,161,162,163,164]. Here, CDKN1A is linked to breast cancer.