1070 Background: Triple negative breast cancer (TNBC) often grows rapidly and has poor prognosis, with a high recurrence rate. Because conventional endocrine treatment and HER2 targeted therapy for TNBC is invalid, chemotherapy is the only systemic treatment for TNBC. It is known that several subtypes within the TNBC show different responses to chemotherapy, depending on the subtypes. Recently, a claudin (CLDN) low breast cancer has been identified, exhibiting low expressions of CLDNs 1, 3, 4 and 7. CLDNs are transmembrane proteins that seal tight junctions and are critical for maintaining cell-to-cell adhesion in epithelial cell sheets. However, their role in cancer progression remains largely unexplored. Methods: Surgically removed, formalin-fixed, paraffin-embedded breast cancers from 341 TNBC patients were analyzed to identify CLDN expression.They underwent wide local excision or mastectomy between March, 2004 and December, 2007 at the breast surgery unit of Asan Medical Central Hospital. Results: In our tumor series, we found 45.0% (153/339) of high expressing cases for CLDN1, 57.0% (192/337) for CLDN3, 57.6% (194/337) for CLDN4 and 44.0% (149/339) for CLDN7. Overall, we found 20.5% (70/341) of cases were within the low CLDN expression group and 79.5% (271/341) of tumors were within the high expression group of CLDN1, 3, 4 ,7. Although the high CLDN expression group was significantly associated with positive lymph node status and higher stage, there were no significant differences between CLDN low and high groups in disease free survival (p=0.477) or overall survival (p=0.253). Conclusions: CLDN high tumors are associated with poor prognosis features, but they are not an independent prognostic factor in TNBC patients. However, the mechanisms underlying the different roles of CLDNs in tumorigenesis are largely unclear. Studying the associations of these CLDNs with the TNBC subgroup of breast cancers might provide us with potential diagnostic biomarkers or therapeutic targets for cancer cells.