ALDH1A1 and triple-negative breast carcinoma: Compared with non-CSC populations, enriched PD-L1 expression was observed in CSC populations (CD44+CD24−/low population in human breast cancer21 and CD44+CD24+ALDH1+ population in mouse breast cancer22) of multiple triple-negative breast cancer (TNBC) cell lines (Supplementary Fig. 1a–c).