CLDN4 and inflammatory bowel disease: The gut barrierconsists of the mucus layer, epithelial layer, and immunoglobulins,which are the first defenses against the invasion of microbes fromthe gut lumen into the mucosa; thus, they may prevent uncontrolledimmune activation.24 The expression ofMUC2, the main protein in mucus, and junction proteins ZO-1, occludin,claudin-4, and E-cadherin is significantly reduced in IBD patients,25−28 resulting in impaired gut barrier function.