Illustration of phenotypic switching in response to chemotherapy. Dynamic changes in the expression of ALDH1 were observed during NAC. Chart A illustrates that there was a switch from ALDH1(−) to ALDH1(+) phenotype (positive switch) in 27% of cases and from ALDH1(+) to ALDH1(−) phenotype (negative switch) in 19% of cases, after four cycles of chemotherapy. However, patients who remained ALDH1(−) both at baseline and after four cycles had the highest response rates, while those who were ALDH1(+) at both time points had the lowest response rates. Those who were ALDH1(+) at any time point had intermediate response rates (B). We observed a positive switch more often on patients receiving docetaxel (TAX) and a negative switch more often in patients receiving FEC chemotherapy (C). Finally, the overall survival of patients who remained ALDH1(−) throughout the eight cycles of NAC was significantly higher than those who were ALDH1(+) at any time point (D). (* = P <0.05 Fishers exact test). ALDH1, aldehyde dehydrogenase-1; FEC, fluorouracil, epirubicin, cyclophosphamide; NAC, neoadjuvant chemotherapy.