Abstract Introduction: The molecular architecture of tight junctions (TJ) has been a subject of extensive studies in a number of malignancies. Among the TJ components, the members of the protein family claudins (CLDN) have been found to be differentially expressed correlating with histological grade (HG) in a number of tissues. Furthermore, the expression of these junctional proteins (JP) have recently been implied in the identification of the claudin-low subtype by gene expression analysis. Material and methods: 261 breast cancer specimens diagnosed between 1999 and 2002 were collected and evaluated histopathologically and immunophenotypically by a single specialist. Tissue microarrays were constructed using a TMA-builder instrument (Histopathology Ltd., Pecs, Hungary). To identify the eventual prognostic role of the TJ proteins CLDN3, −4 and −7, and adherent junction protein E-cadherin (ECDH) their immunohistochemical (IHC) expression was analysed by 3 investigators separately, and a consensus score was reached. The IHC results were compared to the patients' follow-up data applying Kaplan-Meier analysis supported by log-rank test. Furthermore, we aimed at identifying the worse prognosis tumour group based on clustering of the expression of the above mentioned JPs. Results: The expression of CLDN3 correlated with vascular invasion (VI)/p = 0.029/, necrosis (NC)/p = 0.001/, HG/p = 0.001/ and mitotic index (MI)/p = 0.022/. CLDN4 and CLDN7 correlated with NC/p = 0.001, p = 0.012, respectively/ and HG/p = 0.034, p = 0.001, respectively/, while ECDH correlated with HG/p = 0.001/ and MI/p = 0.001/. In single protein analysis, groups displaying gradually increasing CLDN3, −7 and ECDH/p = 0.017/expression (split at mean expression ± SD) showed a trend of bearing poorer prognostic features. CLDN4, on the other hand, showed different tendency: gradually increasing expression also correlated with worse relapse-free survival (RFS), while CLDN4 negative/low tumors had the poorest prognosis/p = 0.034/. In multiple protein analysis, groups were created using k-means clustering and the following cohorts were created - appearing in prognostic sequence starting with the longest RFS: ECDH-low groups with 1) low/n=31/ and 2) high/n=42/ CLDN3 expression; 3) CLDN7-low (CLDN3, -4 and ECDH-high) group/n=35/, 4) “all-high” expression group/n=70/; 5) CLDN3-low (CLDN4, -7 and ECDH-high) group/n=68/ and 6) “all-low” group/n=15/. The latter was characterised by the CLDN4-low feature as compared to the other groups. Conclusion: Expression of JPs can have prognostic relevance in breast carcinomas. The groups with low expression of ECDH have good prognosis, while gradually increasing JP expression outlines poor prognostic tumours. Strikingly the drop in all JPs expression - most likely resembling the claudin-low subtype - specifies the subgroup with the poorest outcome, which still remains an infrequent disease entity. Grant support: TÁMOP-4.2.2/B-10/1-2010-0013. Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P3-05-08.