Recent advances in the understanding of cell cycle control by cyclins and cyclin-dependent kinases provide a basis for delineating the molecular mechanisms of proliferation control by steroids and the development and progression of hormone-dependent cancers. Cyclin D1 is necessary, rate-limiting and sufficient for G1 progression in breast cancer cells and regulation of cyclin D1 expression or function is an early response to steroid and steroid antagonist regulation of proliferation. The cyclin D1 gene is amplified in approximately 15%, and its product overexpressed in 40-50%, of primary breast carcinomas. The strong evidence that cyclin D1 plays a major role in cell cycle control in breast epithelial cells suggests that its deregulated expression may have effects on disease progression and phenotype including sensitivity to endocrine therapies.