any functional anomaly of the mammary gland epithelium which is normally enveloped by a basement membrane, embedded into fatty connective tissue stroma, and organized as a bilayer, with an inner luminal layer of secretory epithelial cells, and an outer basal layer; each cell layer expresses specific molecular markers; luminal epithelial cells are characterized by the expression of the receptors for systemic hormones, i.e. estrogens, progesterone and prolactin, whereas basal cells (aka myoepithelial cells) express smooth muscle contractile proteins; in functionally differentiated mammary gland, during lactation, luminal cells produce milk, whereas myoepithelial cells are contractile and serve for milk expulsion