The development of biological tools in breast cancer has allowed the classification of prognostic sub-types, and consequently the individualisation of treatments. Molecular classification may be one of these tools. An understanding of the physiology of the mammary gland forms the basis of these classifications. Therefore, within the epithelials cells of the mammary gland we find luminal, myoepithelial and progenitor cells. Luminal cells express hormone receptors (oestrogen and progesterone), some cytokeratins (CK8, CK18), as well as certain specific transcription factors such as GATA3 and FOXA1. Myoepithelial cells express other cytokerartines (CK14) and, when well differentiated, smooth muscle markers (smooth muscle actin).