BMI1 and head and neck squamous cell carcinoma: More specifically, elevated Bmi-1 expression is predominantly observed in ALDHhighCD44+ when compared to ALDHlowCD44− cells in vitro and in vivo (19, 70), and endothelial cell-secreted factors further induce Bmi-1 expression in the CSCs (19), revealing Bmi-1 as an important player in HNSCC CSC biology.