The cyclin D1 protein is involved in promoting transition from the G1 to the S-phase in the cell-cycle by binding to cyclin-dependent kinase 4 (CDK4) or CDK6 proteins, and its overexpression enhances the activity of ERα and counteracts the breast cancer-suppressor gene breast cancer 1, early onset (BRCA1) [66]. The gene discussed is ESR1; the disease is breast cancer.