At least four cell types in mouse mammary epithelium, three in human, and three in cow are now known to be proliferation competent. Some evidence indicates that pregnancy may confer proliferative competency on a new cell type. These cells are widely seen as stem and progenitor cells that maintain the epithelium and produce lactational units during pregnancy. Evolutionarily conserved developmental signaling pathways active in germinal and neuronal stem cell proliferation and differentiation in drosophila and mammalian development are implicated in mammary tumorigenesis. In adult tissues this signaling is retained, is regulated by stem cell niches and operates to create new tissue and maintain tissue form and integrity. Disruption of this signaling may abrogate maintenance of the stem cell niche and lead to preneoplastic conditions.