Tight junctions (TJs) are intercellular structures in epithelial and endothelial cells, primarily playing �critical roles in cell–cell adhesion. Among their molecular components, claudins are the main constituents as integral membrane proteins, encoded by at least 24 members of a single gene family. Accumulated �evidence has demonstrated that TJ proteins such as claudins are directly involved in the regulation of �cellular functions such as proliferation, differentiation, and apoptosis, due to their ability to recruit various signaling molecules that have proliferative and differentiative capacities, including transcription factors, lipid phosphatases, and cell cycle regulators. It is thus clear that TJs are not simple static constituents to establish cell adhesion structures, rather also functioning in cell signaling component that has functions in receiving environmental cues and transmitting signals inside the cells.
