A mere few hundred of the cells from a small sub-population of CD44+/CD24neg/low/ESA+ derived from human breast cancer cells (grown in immuno-compromised non-obese diabetic/severe combined immunodeficiency; NOD/SCID mice) were capable of forming new tumors when injected into immuno-compromised NOD/SCID mice, leading to the identification of tumor-initiating/tumorigenic breast cancer cells [131]. This evidence concerns the gene CD44 and breast cancer.