CDK2 and breast cancer: Studies in breast cancer have highlighted CDK2 as an essential regulator of estrogen-mediated G1/S transition (Planas-Silva and Weinberg, 1997; Cariou et al, 2000); however, recent studies in non-breast cancer cell lines and in knockout mice have questioned the role of CDK2 in cancer cell proliferation (Berthet et al, 2003; Ortega et al, 2003; Tetsu and McCormick, 2003).