Overexpression of claudin proteins in the metastatic breast cancer cell lines MCF-7 and MDA-MB-415. Three breast cancer cell lines (MCF-7, MDA-MB-415 and MDA-MB-157) and the normal breast epithelial cell line, HMEC were subjected to immunoblot analysis using antibodies to claudin-1, −3, and −4. Overexpression of all three claudin proteins, particularly claudins-3 and −4, was observed in the cancer cell lines relative to the HMEC cells. HMEC, human mammary epithelial cells; cldn, claudin.