During development, select epithelial cells must undergo asymmetric division in order to generate a stratified epithelium. Previous studies have shown that basally positioned epithelial stem cells orchestrate this process, but the mammary epithelium has two distinct cell layers inside the basement membrane, so it remains unclear which cell type is responsible for stratification. Using elegant time-lapse imaging of three-dimensional mouse mammary epithelial cultures, Andrew Ewald and colleagues now reveal (p. 1085) that it is the apical luminal epithelial cells that divide vertically to generate the stratified mammary epithelium. The authors show that this is accompanied by the loss of tight junctions, as marked by ZO-1, as well as loss of apicobasal polarity in the new daughter cells. Importantly, the authors also demonstrate that this mechanism of stratification and loss of polarity operates during early oncogenesis in the mouse mammary epithelium. These data uncover a common cellular mechanism that underpins the developmental and oncogenic stratification of mammary tissue.