Esophageal cancer is one of the most common malignancies in the world.In order to identify the proteinsassociated with esophageal squamous cell carcinomas(ESCC),we analyzed the protein profiles of ESCC cases withtumor and matched adjacent normal tissues.Methods Two-dimensional electrophoresis(2-DE)was carried out to analyze the protein profiles.Dysregulated proteinspots were identified by Matrix-Assisted Laser Desorption Ionization Time-of-Flight(MALDI-TOF)and verified by liquidchromatography/electrospray ionization ion trap-mass spectrometry/mass spectrometry(LC-ESI-IT MS).RT-PCR andimmunohistochemistry on tissue microarray were performed to confirm the gene dysregulation in esophageal canceroustissues.RNA interference(RNAi)was used to knock down the gene expression in ESCC cell lines.Apoptosis assay withannexin V-FITC/PI staining was conducted and cells were analyzed by flow cytometry.Results 2-DE showed that two protein spots with approximate molecular weights and different pl were elevated in 12out of 18 ESCCs as compared to the corresponding normal tissues.Both the two spots were identified as MnSOD byMALDI-TOF and were verified by LC-ESI-IT MS.MnSOD overexpression was detected in 14 tumors out of 24 cases byRT-PCR and 52 tumors out of 116 cases by immunohistochemistry comparing to normal epithelia,siRNA-mediatedsilencing of MnSOD in KYSE450 and KYSE150 cell lines revealed that MnSOD protected ESCC cells from apoptosisinduced by ultraviolet(UV)and doxorubicin(DOX).Conclusions These findings suggest that there existed two isoforms of MnSOD protein in normal and tumoresophageal tissues.MnSOD was overexpressed in ESCC and its up-regulation in esophageal cancer cells wasassociated with apoptosis resistance. Esophageal cancer is one of the most common malignancies in the world. In order to identify the proteinsassociated with esophageal squamous cell carcinomas (ESCC), we analyzed the protein profiles of ESCC cases with tumor and matched adjacent normal tissues. Methods Two-dimensional electrophoresis -DE) was carried out to analyze the protein profile. Dysregulated proteins spots were identified by Matrix-Assisted Laser Desorption Ionization Time-of-Flight (MALDI-TOF) and verified by liquid chromatography / electrospray ionization ion trap-mass spectrometry / mass spectrometry -ESI-IT MS). RT-PCR and immunohistochemistry on tissue microarray were performed to confirm the gene dysregulation in esophageal cancerous tissue. RNA interference (RNAi) was used to knock down the gene expression in ESCC cell lines.Apoptosis assay withannexin V-FITC / PI staining was conducted and cells were analyzed by flow cytometry. Results 2-DE showed that two protein spots with approximate molecular weights and different pl were elevated in 12 out of 18 ESCCs as compared to the corresponding normal tissues.Both the two spots were identified as MnSOD byMALDI-TOF and were verified by LC-ESI-IT MS. MnSOD overexpression was detected in 14 tumors out of 24 cases by RT-PCR and 52 tumors out of 116 cases by immunohistochemistry comparing to normal epithelia, siRNA-mediated silencing of MnSOD in KYSE450 and KYSE150 cell lines revealed that MnSOD protected ESCC cells from apoptosis led by ultraviolet (UV) and doxorubicin (DOX) .Conclusions These findings suggest that there existed two isoforms of MnSOD protein in normal and tumoresophageal tissues. MnSOD was overexpressed in ESCC and its up-regulation in esophageal cancer cells wasassociated with apoptosis resistance.