In 2003, Al-Hajj and colleagues identified a population of cells in human breast cancers with a Lin−ESA+CD44+CD24−/low phenotype that were highly tumourigenic in vivo and were the only cell population capable of forming tumours in NOD/SCID mice that recapitulated the heterogeneity of the tumour from which they were isolated [17]. This evidence concerns the gene CD44 and breast carcinoma.