The down-regulation of both claudins (siCldn3 + siCldn4) led to a significantly reduced responsiveness of the colon cancer cells towards recCPE treatment and optCPE gene transfer compared to control (siCo) treated cells, demonstrating specific activity of CPE to its receptors claudin-3 and -4 (Additional file 2). Here, CLDN3 is linked to malignant colon neoplasm.