Accurate assessment of malignancy in oral squamous cell carcinoma is essential to optimize treatment planning. To detect a biomarker related to malignant propensity in gingival squamous cell carcinoma (GSCC), quantitative gene expression analysis of tetraspanin family genes was conducted. In 73 cases of GSCC, total RNA was extracted from carcinoma tissues, and gene expression was analyzed by quantitative real time-PCR. Six tetraspanin family genes (CD9, CD63, CD81, CD82, CD151, NAG-2) were investigated. Housekeeping genes (ACTB and GAPDH), anchor protein genes (JUP and PXN) and an integrin gene (ITGA3) were used as reference genes. Forty-five gene expression ratios were calculated from these 11 gene expression levels and were analyzed with clinical parameters using multivariate statistical methods. According to the results of the logistic regression analysis subjecting cervical lymph node metastasis as a target variable, CD9/ACTB (p 5 0.013) or CD9/CD82 (p 5 0.013) in addition to tumor size (p 5 0.028) were detected as significant factors. In Cox proportional hazards regression analysis, delayed cervical lymph node metastasis (p 5 0.039) and tumor cell positive surgical margin (p 5 0.032) in addition to CD151/GAPDH (p 5 0.024) were detected as significant factors for death outcome. A Kaplan-Meier survival curve presented a significantly lower survival rate of the group with a CD151/GAPDH value of 10 or more (log rank and generalized Wilcoxon tests: p 5 0.0003). Results of this study present the usefulness of CD9 and CD151 expression levels as biomarkers for assessment of malignancy in GSCC. They also indicate that detection of residual tumor cells at the surgical margin and the biological malignancy of a tumor interdependently affects prognosis. ' 2009 UICC Key words: biomarker; tetraspanin; CD9; CD82; CD151; gingival squamous cell carcinoma; lymph node metastasis; death outcomeThe incidence of squamous cell carcinoma (SCC) arising from gingiva is relatively high among oral malignant neoplasms, and approximately half of the cases also presented with cervical lymph node (LN) metastasis. Because gingival SCC (GSCC) easily invades the jawbone and its surrounding tissues such as nerve, muscles, the nasal cavity and skin, determination of the extent of resection and reconstruction are significant issues in treatment of advanced cases.1-3 Therefore, determination of optimal treatment plans based on accurate assessment of biological malignancy is extremely important not only in terms of controlling cancers but also for minimizing dysfunction after treatment. Candidates for biomarkers of head and neck cancers including oral SCC (OSCC) were found in some studies; however, they have not been of practical use. 4,5 Although diagnostic imaging, status of lymph node metastasis and histology still will be clinically essential criteria, they are not able to provide sufficient information to realize the individualized treatment of OSCC in the future. A precise malignancy diagnosis of OSCC, such as prediction of potential metast...