Previous studies have shown that the Sirt1 can repress the expression of p21 through the transcriptional inactivation of p21 promoters in breast cancer cells, vascular smooth muscle cells and human gingival fibroblasts.53-55 The overexpression of p21 can lead to cell cycle arrest and activate Caspase 3, which in turn can mediate apoptosis. The gene discussed is CDKN1A; the disease is breast cancer.