Aim: The purpose of this study was to investigate the expression of MLH-1 and PTEN proteins in oral squamous cell carcinomas( OSCC) and precancerous lesions,discuss the significance,and analyze the correlation between them. Methods: Immunohistochemical SP method was used to detect the expression of MLH-1 and PTEN in 48 cases of oral squamous cell carcinoma,11 cases of simple hyperplasia,10 cases of DYSplasia,and 10 cases of normal oral mucosa. Results: The positive rate of MLH-1 in48 cases of OSCC( 47. 9%) ...