Common phenotypes of cancer and stem cells suggest that breast cancers arise from stem cells. Breast epithelial cells with stem cell phenotypes have been shown to be more susceptible to immortalization and neoplastic transformation. Breast tumor stem cells with CD44+/CD24-/lowLineage- markers have been isolated. The role of these cells in tumor progression and clinical outcome is not clear. The relationship between breast stem cell and tumor stem cell may be elucidated by further studies of carcinogenesis of nonadherent mammosphere cells with stem cell features and by derivation of CD44+/CD24-/low cells from an adherent breast epithelial stem cell type.