['Claudins are 22-27 kDa sized adhesion molecules that constitute tight junctions. Their expression levels are often tissue-specific, and their altered degrees of expression have been reported in a variety of cancers. In addition, the prognostic significance of claudin expression has been implicated in various human cancers. However, the prognostic significance of claudin-4 expression in esophageal squamous cell carcinoma (ESCC) remains to be clarified.', 'We investigated the prognostic significance of claudin-4 expression in 164 cases of ESCC using immunohistochemisty. We also evaluated claudin-4 mRNA expression levels using quantitative real-time reverse transcription polymerase chain reaction (RT-PCR) and analyzed its specific promoter methylation status using quantitative methylation-specific PCR.', 'According to clinicopathological parameters, low claudin-4 expression was found to be significantly associated with histological differentiation (P = 0.003), invasion depth (P = 0.002), and lymph node metastasis (P = 0.024). Low claudin-4 expression showed unfavorable influences on disease-free survival (P = 0.0115) and overall survival (OS) (P = 0.0009). In multivariate analysis, low claudin-4 expression was an independent predictor of poor OS (P = 0.007). Claudin-4 mRNA levels assessed using real-time RT-PCR were consistent with the protein levels determined using immunohistochemistry. Furthermore, this study demonstrates that loss of claudin-4 is associated with promoter hypermethylation.'] Is low expression of claudin-4 associated with poor prognosis in esophageal squamous cell carcinoma?