The first insight indicating a role of breast cancer stem cells (BCSC) in breast carcinogenesis was demonstrated by the study of Al-Hajj and co-workers, where Lin-CD44+CD24-/low cells injected into the mammary fat pad of non-obese diabetic/severed combined immunodeficient (NOD/SCID) mice were able to form palpable tumors, even with just a few hundred of cells being transplanted. This evidence concerns the gene CD24 and breast cancer.