Breast cancer is a genetically and clinically heterogeneous disease. It is unclear whether different target cells contribute to this heterogeneity and which cell types are most susceptible to oncogenesis. Stem cells are speculated to be the cellular origin of at least a subset of human breast cancers. To begin to address these issues, we have isolated and characterized cell populations enriched in normal mammary stem/progenitors and have studied the expression of putative stem/progenitor markers in tumors derived from genetically engineered mice. Specifically, transgenic activation of Wnt signaling in the mammary gland induces tumors comprised of epithelial and myoepithelial cells harboring the same genetic defect implying that the tumor arose from transformation of a bipotent progenitor cell. On the other hand, transgenic activation of Neu signaling induces tumors comprising cells of more limited lineage capacity. Thus, the heterogeneity of different breast cancers may reflect the activation of different oncogenic pathways, different cellular targets in which these genetic changes occur, or both.