Expression of Bmi-1 protein in tissue by immunohistochemistry. A and B, only weak staining of Bmi-1 was detected in few normal esophageal epithelial tissue (arrow, normal epithelial cells). C and D, positive expression of Bmi-1 in esophageal carcinoma tissues (200× and 400× of magnification, respectively). E and F, representative case of higher expression of Bmi-1 in invasive front (arrow, 200× and 400× magnification, respectively). Bmi-1 expression mainly localized in nuclei of tumor cells.