CD44 and breast cancer: Furthermore, in Id1-overexpressing MCF7 luminal type breast cancer cells, the CD44+/CD24−/ESA+ breast CSC population was enriched, whereas short hairpin RNAs (shRNAs)-mediated Id1 knockdown in MDA-MB-231 basal-like type breast cancer cells reduced the size of this population (Figures 4A, bottom panel, and 4B).