CLDN1 and colorectal carcinoma: In fact, claudin members are often highly expressed in a variety of gastrointestinal tumors, for example, claudin-3, -4, and -7 are highly expressed in early esophageal adenocarcinoma [9], claudin-1 is highly expressed in gastric cancer [10], and claudin-1 and -2 are highly expressed in colorectal cancer (CRC) [11].