BMI1 and head and neck squamous cell carcinoma: Krishnamurthy et al. (2010) and Zhang et al. (2014) showed that ALDH+ cells in HNSCC are located in close proximity to blood vessels [6,49] where endothelial cell-derived growth factor enhances the expression of Bmi-1, increases spheroid formation, and enhances tumour growth in vivo; selective ablation of blood vessels reduces the proportion of CSCs in vitro and in vivo, suggesting that endothelial cells are able to secrete factors that promote the proliferation, survival, and self-renewal of HNCSCs [28,49].