CDKN1B and breast cancer: Because anti-estrogen therapy results in a reduction in cyclin D1 levels and p21Waf1/Cip1/p27Kip1-mediated inhibition of CDK2/1 activity, we sought to determine the ability of novel guanine-based CDK2/1 inhibitors NU2058 and NU6102 to target CDK2 and CDK1, and assess their therapeutic potential as alternatives to hormonal therapy in breast cancer.