Elucidation of the genes controlling the proliferation and differentiation of mouse mammary epithelial stem (MaSC) and progenitor (Ma-CFC) cells is paramount to understanding the processes that regulate mammary gland development and breast cancer progression. We have previously described a strategy in which MaSC and Ma-CFC can be purified to 5% and 15%, respectively, on the basis of lack of expression of the hematopoietic and endothelial markers CD45, Ter119 and CD31 and on the differential expression of CD24 and CD49f 1, with the MaSC having a CD24^medCD49f^high phenotype and the Ma-CFC having a CD24^highCD49f^low phenotype. Currently, a definitive analysis of the gene expression profiles of MaSC and Ma-CFC is not possible due to the presence of large numbers of contaminating cells in these enriched subpopulations. However, a preliminary microarray analysis of these subpopulations has identified potential new cell surface markers that can be exploited to further purify MaSC and Ma-CFC. We have initiated a screening program using the markers identified in the microarray analysis as well as markers used to identify other adult tissue stem cells to further purify and characterize MaSC and Ma-CFCs. Results of this screen will be presented.