ALDH1A1 and cancer: CSCs are also implicated in cancer recurrence and therapy resistance.[26, 27] Therefore, we further subdivided the epithelial cells into 17 clusters through dimensionality reduction (Figure 2A), and identified CSC subpopulations in cluster 2 that originated from primary tumor and clusters 3, 5 and 16 originated from recurrent tumor, based on the expression of CSCs markers CD44, ALDH1A1, and SOX4 (Figure 2B; Figure S1G, Supporting Information).