Although two cell lines (BT20 and AU565) were relatively insensitive to cyclin D1 siRNA (< 25% inhibition) but sensitive to PD0332991, and one cell line, MDA-MB-436, was resistant to PD0332991 but showed response to cyclin D1 knockdown (Additional file1: Table S3), overall sensitivity to PD0332991 was significantly correlated with sensitivity to cyclin D1 knockdown (R = 0.65, P = 0.0003, Figure 3A), confirming cyclin D1 as a major activator of CDK4/6 in regulation of breast cancer cell cycle progression. This evidence concerns the gene CDK4 and breast cancer.