The estrogen receptor α (ERα), progesterone receptor (PR), and erbB2 (Her2 in humans) are important prognostic markers of human breast cancer, and they are variably expressed in different subtypes of breast cancer. The basal subtype, for example, is negative for ERα, PR, and Her2 by immunohistochemistry. We investigated the expression of these signaling molecules in enriched populations of mouse mammary stem cells and luminal cells that were isolated according to their differential expression of CD24 and the α6β1-integrin complex. We found that the basal population, which is enriched in mouse mammary stem cells, did not express ERα, PR, or ErbB2/Her2 but did express epidermal growth factor receptor (EGFR)/ErbB1, whereas the subset of cells enriched for luminal cells expressed ERα (37% of cells) and PR (40% of cells) but not ErbB2/Her2 or EGFR/ErbB1. Ovariectomy confirmed the importance of estrogen signaling to luminal cell proliferation but had no effect on the size of the mouse mammary stem cell-enriched population. Thus, mouse mammary stem cells were negative for ERα, PR, and ErbB2 and appeared to share common properties with poor-prognosis basal breast cancer.