FBXW7 is reported to be a tumor suppressor gene, and the functional inactivation of FBXW7 has been reported in various human tumors. In this study, we investigated the FBXW7 gene in human thymoma: although no mutations were evident, a significantly high frequency of methylation in the FBXW7 beta-form promoter was observed in types B1 or higher (P = 0.014). We propose a novel mechanism for the pathogenesis of thymoma by FBXW7 beta-form and hypothesize that expressional suppression plays an important role in the malignant potential of thymoma. (C) 2008 Elsevier Inc. All rights reserved.