We previously showed that breast cancer cases with core basal phenotype (CBP; estrogen receptor negative [ER−], progesterone receptor-negative [PR−], human epidermal growth factor receptor 2-negative [HER2−], cytokeratins (CK 5 or CK5/6)-positive [CK5/6+] and/or epidermal growth factor receptor-positive [EGFR+]) tumors had significantly reduced TDLU involution compared with cases with luminal A (ER+ and/or PR+, HER2−, CK5/6−, EGFR−) tumors from a population-based case-control study in Poland. This evidence concerns the gene KRT5 and breast cancer.