CLDN1 and colorectal carcinoma: Based on the currently available body of work on the protein expression of claudins in colorectal cancer, CLDN1, CLDN2, CLDN4, and CLDN18 have all been reported to be expressed in CRC [27,28,29,30,31,32,33,34,37,46,56,57,58,59,60,61,62,63,73,74,75,80,81,93,96,152,153,160,161,162,163,164].