ALDH1A1 and breast cancer: Consistent with the above results, the ALDH1-positive subpopulation showed a significantly higher level of TCF-4 (a positive regulator of Wnt/β-catenin signaling) compared with that in the ALDH1-negative subpopulation in two different breast cancer cell types (Fig. 2B), suggesting that the BCSC subpopulations exhibited enhanced Wnt/β-catenin signaling activity.