CDK4 and breast cancer: Human colorectal and breast cancer cells treated with Nar exhibited a marked down-regulation in the mRNA of genes related to cell cycle control, including Cdk4, Cdk6 and Cdk7. However, the same treatment resulted in the upregulation of cell cycle-dependent kinase inhibitor genes, p18, p19 and p21. These results are consistent with the findings in other types of malignancies [31,42].