The recombinant C-CPE-ETA' fusion protein was shown to retain the specificity of binding to CLDN-4 and initiate rapid penetration into cytosol in five different CLDN-4-positive cancer cells (MCF7, A431, SW480, PC3, and DU145) but not to CLDN-4-negative cells (HELA, HUVEC). This evidence concerns the gene CLDN4 and cancer.