Previous studies have shown that in both normal [15] and tumor cell lines [16], the introduction of p21 expression constructs result in a cell cycle arrest at the G1 phase [17, 18]; p21 has an effective inhibitory capacity in the G1/S transition CDKs including CDK2, CDK3, CDK4, and CDK6 [18]. The gene discussed is CDKN1A; the disease is neoplasm.