CD44 and breast carcinoma: It has been reported that tumorigenic breast cancer stem cells are enriched with a sub-population of cells expressing CD44+/CD24-/low/ESA+ on their surface [39], and that a sub-population expressing a similar combination of cell-surface markers has been identified in breast cancer cell lines (such as MDA-MB-231), and has been shown to possess high capacity for self renewal and tumor initiation [40].