Published studies evaluating the role of cyclin D1 in breast cancer support the current observations of the activity of a CDK4/6 inhibitor in luminal ER-positive breast cancer, its synergism with tamoxifen in cell lines that are sensitive to hormone manipulation, as well as the reversal of resistance of those that have acquired a resistant phenotype in the face of anti-estrogen therapy. This evidence concerns the gene CCND1 and breast cancer.