Claudin-9 is a protein that is part of the claudin family, which plays a crucial role in forming tight junctions (TJs) in epithelial and endothelial cells. Tight junctions are essential for maintaining the barrier function of cell sheets, regulating the paracellular transport of molecules, and providing a scaffold for signaling molecules. Claudin-9 has been implicated in various cancers. It is expressed in several types of tumors and is associated with poor prognosis in gastric cancer. It is also linked to the glycolytic levels in endometrial and esophageal adenocarcinomas and is being investigated for its potential as a therapeutic target in cancer treatment.