This discovery comes from the observation that ALL cells are organized hierarchically, with a subset of cells that can replicate themselves and give rise to other malignant lineages, mimicking normal hematopoietic stem cells.2 In solid tumors, CSCs were first identified in breast cancer, in which the CD44+ CD24−/low lineage-cells underwent self-renewal and differentiation processes.23 These two properties of CSCs are also the basis to explain the formation of intratumoral heterogeneity in the CSC hypothesis or the hierarchical model of tumorigenesis4. This evidence concerns the gene CD44 and breast cancer.