For example, mutations in CDKN1A (known as p21), which is one of the best-characterized direct target genes of TP53 and prevents cell cycle progression (Abbas and Dutta, 2009), are much less frequent than mutations in TP53. Moreover, Cdkn1A knockout mice have a much lower tumor penetrance than TP53 knockout mice (Martin-Caballero et al., 2001), suggesting that additional TP53 targets must contribute to tumor suppression (Brady et al., 2011). This evidence concerns the gene CDKN1A and neoplasm.