The Bmi-1 oncoprotein regulates proliferation and oncogenesis in human cells. 
Its overexpression leads to senescence bypass in human fibroblasts and 
immortalization of human mammary epithelial cells. In this study, we report that 
compared with normal nasopharyngeal epithelial cells (NPEC), Bmi-1 is 
overexpressed in nasopharyngeal carcinoma cell lines. Importantly, Bmi-1 was 
also found to be overexpressed in 29 of 75 nasopharyngeal carcinoma tumors 
(38.7%) by immunohistochemical analysis. In contrast to nasopharyngeal 
carcinoma, there was no detectable expression of Bmi-1 in noncancerous 
nasopharyngeal epithelium. Moreover, high Bmi-1 expression positively correlated 
with poor prognosis of nasopharyngeal carcinoma patients. We also report that 
the overexpression of Bmi-1 leads to bypass of senescence and immortalization of 
NPECs, which normally express p16(INK4a) and exhibit finite replicative life 
span. Overexpression of Bmi-1 in NPECs led to the induction of human telomerase 
reverse transcriptase activity and reduction of p16(INK4a) expression. 
Mutational analysis of Bmi-1 showed that both RING finger and helix-turn-helix 
domains of it are required for immortalization of NPECs. Our findings suggest 
that Bmi-1 plays an important role in the development and progression of 
nasopharyngeal carcinoma, and that Bmi-1 is a valuable marker for assessing the 
prognosis of nasopharyngeal carcinoma patients. Furthermore, this study provides 
the first cellular proto-oncogene immortalized nasopharyngeal epithelial cell 
line, which may serve as a cell model system for studying the mechanisms 
involved in the tumorigenesis of nasopharyngeal carcinoma.