Claudin is a transmembrane protein with four transmembrane domains. Claudin-7 (Cldn7) is a member of the claudin family, and its basic function is to maintain cell polarity and form tight junctions. CLDN7 has a full-length form (211 amino acids) and a C-terminal truncated form (158 amino acids). Recent studies have shown that low expression of the Cldn7 protein is found in a variety of malignant tumors, such as lung cancer, breast cancer, pancreatic cancer, and colorectal cancer (CRC).