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Background X-linked inhibitor of apoptosis (XIAP)-associated factor 1 (XAF1) is a new tumor suppressor.Low expression of XAF1 is associated with poor prognosis of human cancers.However,the effect of XAF1 on lung cancerremains unknown.In this study,we investigated the expression of XAF1 and its role in squamous cell lung cancer.Methods Cancer tissues,cancer adjacent tissues and normal lung tissues were collected from 51 cases of squamous cell lung cancer.The expression of XAF1 mRNA was determined by reverse transcription-polymerase chain reaction (RT-PCR).The expression of XAF1 protein was determined by Western blotting and immunohistochemical staining.Ad5/F35-XAF1 virus was generated.Cell proliferation and apoptosis were measured by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) method and flow cytometry (FACS),respectively.Results The levels of XAF1 protein and mRNA in cancer tissues were significantly lower than those in cancer adjacent and normal lung tissues (P ＜0.05).The low expression of XAF1 was associated with tumor grade,disease stage,differentiation status and lymph node metastasis in squamous cell lung cancer patients.The restoration of XAF1 expression mediated by Ad5/F35-XAF1 virus significantly inhibited cell proliferation and induced apoptosis in a dose- and time-dependent manner.Conclusion XAF1 is a valuable prognostic marker in squamous cell lung cancer and may be a potential candidate gene for lung cancer therapy.