The study by Dontu et al. describes the development and characterization of an in vitro cultivation system for human mammary epithelial cells (HMECs) that allows for the propagation of these cells in an undifferentiated state as nonadherent mammospheres. This system overcomes the limitations of traditional methods that rely on cell surface markers and in vitro assays for testing stem cell properties. The authors demonstrate that nonadherent mammospheres are enriched in early progenitor/stem cells and can differentiate along all three mammary epithelial lineages. Gene expression analysis of cells isolated from nonadherent mammospheres reveals overlapping genetic programs with other stem and progenitor cells, identifying new markers that may be useful for the identification of mammary stem cells. The isolation and characterization of these stem cells are expected to help elucidate the molecular pathways governing normal mammary development and carcinogenesis. The study also highlights the potential of this culture system for studying the self-renewal and differentiation of human mammary stem cells.The study by Dontu et al. describes the development and characterization of an in vitro cultivation system for human mammary epithelial cells (HMECs) that allows for the propagation of these cells in an undifferentiated state as nonadherent mammospheres. This system overcomes the limitations of traditional methods that rely on cell surface markers and in vitro assays for testing stem cell properties. The authors demonstrate that nonadherent mammospheres are enriched in early progenitor/stem cells and can differentiate along all three mammary epithelial lineages. Gene expression analysis of cells isolated from nonadherent mammospheres reveals overlapping genetic programs with other stem and progenitor cells, identifying new markers that may be useful for the identification of mammary stem cells. The isolation and characterization of these stem cells are expected to help elucidate the molecular pathways governing normal mammary development and carcinogenesis. The study also highlights the potential of this culture system for studying the self-renewal and differentiation of human mammary stem cells.