Proposed epithelial cell hierarchy in the mouse and human mammary glands. Basal stem cells undergo either self-renewal or differentiation into a luminal progenitor (during embryogenesis) or a myoepithelial cell. Upon certain environmental signals, the luminal progenitor populations may be able to commit to an oestrogen receptor (ER)-positive ductal cell or, during pregnancy, to a milk-producing alveolar cell. In vivo lineage tracing experiments in mice will need to be performed to validate these developmental relationships. ALDH, aldehyde dehydrogenase; EpCAM, epithelial cell adhesion molecule; PR, progesterone receptor.