MKI67 and infection: Certainly silencing expression of both p53 and pRb bypassed N-RASinduced cell cycle arrest and senescence in this cell type (15 days after infection only 5% of N-RASQ61Kmelanocytes showed positive Ki67 staining, compared to 24% of N-RASQ61Kmelanocytes lacking both p53 and pRb and 23% of melanocytes expressing onlycontrol shRNA/copGFP; Figure 3A).