To determine whether the high or low expression of the claudin-4 gene detected by microarray and q-RT–PCR assays in primary USPC and NEC cell lines, respectively, is the result of a selection of a subpopulation of cancer cells present in the original tumour, or whether in vitro expansion conditions may have modified gene expression, we performed immunohistochemical analysis of claudin-4 protein expression on formalin-fixed tumour tissue from the uncultured primary surgical specimens from which the USPC cell lines were derived. Here, CLDN4 is linked to cancer.