The Notch signaling pathway links the fate of one cell to that of the next door cellular neighbor.  The developmental action of Notch is pleiotropic and in general Notch activity is associated with early lineages in a very broad spectrum of tissues and organs.  Consequently the Notch receptor is expressed and controls the fate of many early precursors and indeed stem cells.  Based on this, the Artavanis-Tsakonas Lab developed a series of transgenic mice in which an inducible form of the Cre recombinase has been knocked in, respectively, the Notch 1,2,3 and 4 receptor genes, thus expressing Cre in cells  expressing each receptor paralogue (Sale et al, Nat Cell Bio, 2013).  Using these tools they have been determining cell lineages and in this study they followed cell lineages associated with Notch 2 in the mammary gland.  They thus uncovered two new epithelial lineages that which allow them to revise the classical cellular hierarchies associated with mammary development.  They thus revise current models of mammary epithelial cell hierarchy and reveal a hitherto undescribed mechanism regulating branching morphogenesis.  These results may have implications for the identification of the cell-of-origin of distinct breast cancer subtypes.