Upon phosphorylation, transcription factor E2F is released and activated, allowing progression of G1 to S phase.20,21 Many checkpoints are deregulated in cancer cells, and this is often as a result of alterations of the cell cycle regulatory machinery and cyclin–CDK complexes.22 As shown in Figure 2C, the protein expression of CDK2 was decreased significantly in SFN-treated ZR-75-1 cells. This evidence concerns the gene CDK2 and cancer.