In our previous studies, we isolated CSCs of ESCC using side population (SP) sorting with Hoechst 33342, and our results showed that SP cells shared certain common features of cancer stem cells, such as the ability of self-renewal, highly proliferative and tumorigenic, and expressing several stem cell-related genes, including SOX2, OCT-4, BMI-1 and so on [12, 13]. Here, SOX2 is linked to esophageal squamous cell carcinoma.