The comparison of mRNA and protein level of CLDN6 expression among breast cancer MCF-7 cells, HBL-100 cells (human normal breast cells) and COC1cells (human cervical cancer cells as tissue-specific control) showed that CLDN6 expression was decreased in MCF-7 as compared with HBL-100 and COC1, indicating that reduction of CLDN6 expression had tissue- and cell-specificity, which was consistent with clinical assays (Fig. 1b). Here, CLDN6 is linked to breast cancer.