 The aim of this study is to evaluate the association between several methylated genes and oral cancers. A prospective case control trial is designed with sample size estimated at least 160 cases. It is expected to enroll at least 100 subjects with precancerous lesion or oral cancer, and 60 subjects with normal oral mucosa. This study is approved by the National Taiwan University Hospital Research Ethics Committee. After signing the informed consent, all of the subjects will receive an intraoral examination and the oral epithelial cells will be collected. If abnormal oral lesion is observed, it will be photographed, and the biopsy will be performed to confirm the diagnosis. For methylated gene detections, the gDNA will be extracted from the collected cells and standardized assay will be performed. Clinical sensitivity and specificity for these methylated genes will be evaluated. 