CLDN4 and bladder transitional cell carcinoma: However, in epithelial malignancies overexpressing CLDN4, such as bladder urothelial carcinoma (BUC), colon cancer, gastric cancer, and pancreatic cancer, the barrier function of CLDN4 maintains the tumor microenvironment, retains growth factors, and protects tumors from intratumoral permeation of anti-cancer drugs [3,4,5,6].