CD44 and breast cancer: Further studies demonstrate that compared to non-CSCs, PD-L1 total protein and surface expression was enriched in BCSCs (CD44+/CD24−/low population in human breast cancer and CD44+/CD24+/ALDH1+ population in mouse breast cancer) and this enrichment was regulated in response to EMT through an EMT/ß-catenin/STT3/PD-L1 signaling axis.