CD44 and breast cancer: It is well-established that CD44+/CD24− population has the property of cancer stem cells in breast cancer, including (i) ability to form mammary sphere; (ii) ability to initiate tumor more efficiently in vivo; and (iii) ability to regenerate the whole population (including CD44+/CD24− and CD44+/CD24+ and CD44−/CD24− and CD44−/CD24+) [10].