Breast cancer stem cells: something out of notching?

We and others have established that the developmental Notch receptor signaling pathway is active in breast cancer cell lines, as well as in preinvasive and invasive primary samples. Recently, a role for Notch in regulating the hierarchy of stem and progenitor cells in both normal and cancer epithelium has been elucidated. Because inhibiting the Notch receptor signaling pathway is a possible future breast cancer therapy, here, we review the expression and activity of the different ligands and receptors and summarize the various ways in which the pathway's activity can be inhibited, and the likely effects of inhibition on different tumor cell subpopulations.