Overexpression of p16(INK4a) has been observed when retinoblastoma protein is 
inactivated by high-risk human papillomavirus (HPV) oncoprotein E7. We 
investigated overexpression of p16(INK4a) and HPV infection in cervical squamous 
neoplasia to evaluate the oncogenic potential among various HPV subtypes. The 
high-risk HPV was detected by PCR in 69.8% (37/53), 97.5% (39/40), 91.7% 
(44/48), and 100% (16/16) of cervical intraepithelial neoplasia (CIN)1, CIN2, 
CIN3, and squamous cell carcinoma (SCC), respectively. The p16(INK4a) 
overexpression was investigated immunohistochemically using a 
p16(INK4a)-specific monoclonal antibody (clone E6H4). In high-risk HPV positive 
cases, 32.4% (12/37) of CIN1, 82.1% (32/39) of CIN2, 93.2% (41/44) of CIN3, and 
all (16/16) SCC showed p16(INK4a) overexpression. The incidence of p16(INK4a) 
overexpression was significantly different between CIN1 and CIN2, suggesting 
that the disorder of cell cycle regulation by HPV frequently occurred from CIN2. 
As for CIN1 cases, p16(INK4a) overexpression was observed more frequently in 
HPV16 and HPV52 than in HPV51 and HPV35. Using p16(INK4a) as a bio marker of HPV 
oncogenic activity, we demonstrate that the level of pRb dysfunction by 
high-risk HPV varied from subtypes and was getting more frequent from CIN2.