ALDH1A1 and breast cancer: The CD24−CD44+ subpopulation upon carcinogenic E2 treatment generated as many as six different downstream breast cancer progenitor cell-like subpopulations: CD24−CD44+CD49f+ALDH1+ (9%), CD24−CD44+CD49f+ALDH1- (24%), CD24−CD44+CD49f+ALDH+CXCR4+ (1.3%), CD24−CD44+CD49f+ALDH-CXCR4+ (8%), and CD24−CD44+CD49f+ALDH+CXCR4+NRF1+ (1.3%) (Table 1).