Estrogens and antiestrogens control proliferation of breast cancer cells by regulating the expression of multiple components of the cell cycle machinery including cyclins D1 and A, cdc25a and the cyclin dependent kinase inhibitors p21Waf1/Cip1 (p21), and p27 Kip1 (p27) [4], [5], [6]. The gene discussed is CCND1; the disease is breast cancer.