CLDN6 and breast cancer: On the one hand, the down-regulation of claudin-6 might result in abnormal proliferation, poor differentiation and decrease apoptosis, and played a role in mammary epithelial cell malignant transformation in the breast cancers; On the other hand, the down-regulation of claudin-6 may also lead to dysfunction of tight junctions, resulting in loss of cell-cell adhesion and polarity, causing tumor cells invasion and metastasis.