Hematopoietic Stem Cells have been isolated by their ability to pump out Hoechst 33342 dye and form a distinct population definable by flow cytometry--the Side Population (SP). The membrane pump Bcrp has been identified as the molecular determinant of the SP phenotype. An SP population with Bcrp activity has been defined in a number of tissues, including mouse mammary and human breast epithelium, and it has been proposed that the SP phenotype is a universal stem cell marker. Studies of mouse and human breast SP suggest that the population is undifferentiated but capable of differentiating into epithelial structures of both luminal and myoepithelial lineages both in vitro and in vivo. However, evidence that the SP is enriched for stem cells is, at the moment, only correlative, and there are potentially confounding technical issues. We still await formal proof that the SP contains a stem cell population.