CD44 and breast cancer: We previously identified a human breast cancer stem cell (BCSC) population (a CD44+ CD24−/low lineage− population of human breast cancer cells) that in many human breast tumors is enriched for the ability to drive tumor formation in a mouse xenograft model as compared to the remaining non-tumorigenic cancer cells (NTG cells) within the same breast tumor (Al-Hajj et al., 2003).