Objective:  To study the immunocytochemical expression of the tight junction protein Claudin‐7 in smears from breast carcinomas and correlate with grading, nodal status, locoregional and distant metastases and the cellular cohesion.

Methods:  The material consisted of 52 air‐dried smears from fine needle aspirates of breast carcinomas, both primary and metastatic and smears from seven benign lesions. A primary antibody to Claudin‐7 was used for immunocytochemical staining. The degree of staining was recorded as negative, reduced or full, with full expression meaning equivalent to the staining pattern found in the fibroadenomas used as benign control. Staining intensity and the percentage of stained cells were evaluated. The control smears revealed a strong membrane and cytoplasmic positivity in all luminal epithelial cells. Cellular cohesion was graded as: (1) mainly cohesive groups, (2) groups and single cells and (3) mainly single cells.

Results:  All primary and recurrent/metastatic breast lesions expressed Claudin‐7. Full expression was demonstrated in 46% of the cases. Reduced expression was found in 54%. In cases with reduced expression, the percentage of stained cells were usually high, and no smear showed <50% stained tumour cells. The staining pattern was heterogeneous and always mixed membrane/cytoplasmic. Claudin‐7 expression showed a significant correlation (P < 0.05) with grading, locoregional and distant metastases, nodal involvement and cellular cohesion in invasive carcinomas, but not with tumour size or subtype.

Conclusion:  Reduced expression of Claudin‐7 correlated with higher tumour grade, metastatic disease, including loco‐regional recurrences and with cellular discohesion.