Homogeneity in the signaling state of the G1/S network across cancer cells may explain the somewhat counterintuitive observations that some cancer cells are more dependent on key cell-cycle regulators, such as Cdk2 and CyclinE, than non-transformed cells (Martín et al., 2005, Tetsu and McCormick, 2003, van den Heuvel and Harlow, 1993). This evidence concerns the gene PCNA and cancer.