The identification of adult stem cells in the mammary tissue in the TDLU, that are able to differentiate themselves initially into epithelial glandular precursor cells (CK5 and CK8/18 positive), or into precursor cells of the myoepithelial layer (CK5 and SMA positive) and, subsequently, into mature luminal cells (CK8/18 positive) and mature myoepithelial cells [12,13] (SMA positive), confirmed the distinction of the breast cancer in luminal, basal, HER-2 positive and ‘similar to normal’ breast highlighted by gene expression studies [14,15]. The gene discussed is KRT5; the disease is breast cancer.