In the present study, we analysed the possible relationship of claudin-2, -3, -4, -6, -7, and -17 expression and VM formation in two breast cancer cell lines, aggressive MDA-MB-231 and non-aggressive MCF-7 cells, and the human umbilical vein endothelial cell line (HUVEC). The gene discussed is CLDN2; the disease is breast carcinoma.