For instance, SV40 LT antigen and human papilloma virus oncoproteins E6 and E7 have helped understand the tumor-preventing roles of p53 and pRb, two important cellular tumor suppressors that are often mutated in cancer (Chellappan et al., 1992; Ali and DeCaprio, 2001; Rangarajan et al., 2004). This evidence concerns the gene RB1 and cancer.