However, the possibility of effectively enumerating each CSC population may have been hindered by BC intratumoral heterogeneity coupled with very low numbers of cells and, in addition, by two CSC-related factors: (i) the observation that ALDH1+ CSC reside in the center while CD24−/CD44+ CSC are found at the edge of a primary breast tumor, raising the issue of sample bias, and (ii) the ability of CSC to transition from one phenotype (i.e., ALDH1+ or CD24−/CD44+) to the other [14]. This evidence concerns the gene CD44 and breast cancer.