Expression of cyclin D1 was evaluated in different molecular breast cancer subtypes, and results showed a stronger intensity of positive cyclin D1 staining in the ER positive/PR positive subtype than in triple-negative breast cancer (TNBC) cases, and negative cyclin D1 staining was seen in human epidermal growth factor receptor 2-positive (HER2-positive) molecular subtypes. This evidence concerns the gene ESR1 and breast cancer.