Claudin 1, CLDN1 [191] , a gene which is reported to be ''normally expressed in all the living layers of the epidermis'' [192] , in concert with AQP3, is a key component of the tight junction complexes of the epidermis. Low CLDN1 gene expression was correlated with shorter overall survival in lung adenocarcinoma. Overexpression of CLDN1 was correlated with suppression of cancer cell migration, invasion and metastasis [193] . Hoevel et al. report that re-expression of CLDN1, in breast tumor spheroids, induces apoptosis and they conclude: ''These findings support a potential role of the tight junction protein CLDN1 in restricting nutrient and growth factor supplies in breast cancer cells, and they indicate that the loss of the cell membrane localization of the tight junction protein CLDN1 in carcinomas may be a crucial step during tumor progression'' [194] . Tokes et al.also report that malignant invasive breast tumors are negative Table 1 . Gene names and probe accession number of the 27 probes with genes annotated with functions on cell adhesion, cell-cell communication, tight junction mechanisms and epithelial cell polarity shown in the heat map in Figure 8 . for CLDN1 [195] . As in breast cancer [196] , in which reduced expression correlated with recurrence status, the low expression of CLDN1 and other tight junction proteins seems to contribute to cellular detachment.