Claudin 4/CLDN4 is a member of the Claudins family and is one membrane protein of the key Junctions for Tight Junctions. Claudins membrane proteins are composed of four transmembrane domains, and there is a binding domain at the C-terminal, which can bind to proteins in the inner cytoplasm (such as ZO-1), and play an important role in signal transduction. Outside the cell membrane, there are two rings of different lengths (ECL1 and ECL2), which can be in contact with other nearby rings of the same type, forming a tight connection between cells. The extracellular ring is necessary to maintain tight junction function and the integrity of the epithelial barrier. Tight junctions are intercellular adhesion structures located at the tips of epithelial and endothelial cells. Claudins, as an important component of tight junctions, play a role in regulating epithelial cell permeability and maintaining cell polarity. When Claudins protein mutated, the permeability between cells will be affected, which may lead to the occurrence of diseases. More and more studies have revealed that CLDN4 plays a regulatory role in cancer and may be a potential tumor therapeutic target. In fact, a large amount of evidence has shown that Claudins members play an important role in tumor immunotherapy and have great clinical druggability.