CD24 and breast cancer: The percentage of the CD44+/CD24-/ESA+ cell population was significantly increased in the cycling hypoxia-selected subpopulation in comparison with the parental breast cancer cell lines (3-fold increase in MDA-MB 231 F3 cells, Figure 3a,c; 13-fold increase in BCM2 F3, Figure 3b,c; Figure S1 in Additional file 1).