It is generally assumed that squamous cell carcinoma develops in a stepwise 
manner from normal bronchial epithelium towards cancer by the accumulation of 
(epi)genetic alterations. Several mechanisms including mutations and homozygous 
deletions or hypermethylation of the p16(INK4a) promoter region can cause loss 
of p16 expression. Recent studies suggest overexpression of the polycomb-group 
gene BMI-1 might also down-regulate p16 expression. In this study, we analyzed 
the p16 expression in relation to the methylation status of the p16 promoter 
region of the p16(INK4a) gene and the expression of BMI-1 in bronchial squamous 
cell carcinomas (SCC) and its premalignant lesions. Nine (69%) SCC showed loss 
of p16 expression and 10 (77%) showed expression of BMI-1. Of four p16 positive 
samples two (50%) were BMI-1 positive, whereas among nine p16 negative samples, 
eight (89%) revealed BMI-1 staining. Four (44%) p16 negative samples were 
hypermethylated at the p16(INK4a) promoter region; the other p16 negative tumors 
that showed no hypermethylation revealed BMI-1 staining. Only two premalignant 
lesions showed absence of p16 expression, of which one (carcinoma in situ) was 
hypermethylated at the p16(INK4a) promoter region and the other (severe 
dysplasia) showed BMI-1 expression. In total, 11 precursor lesions (48%) 
revealed BMI-1 expression. In conclusion, the results of this study suggest that 
loss of p16 expression by promoter hypermethylation is inconsistently and occurs 
late in the carcinogenic process at the level of severe dysplasia. To what 
extent overexpression of the polycomb-group protein BMI-1 attributes to down 
regulating of p16 expression remains unclear.