CLDN4 and inflammatory bowel disease: Tumor necrosis factor (TNF-α), interleukin 1β (IL-1β), and interferon γ, all playing a role in IBD (Guan and Zhang, 2017; Ramos and Papadakis, 2019) and IBD animal models (Lyons et al., 2018; Yan-Hong et al., 2018), downregulate claudin-3 and claudin-4 expression by epithelial cells, whereas IL-23 and IL-9 downregulate claudin-8 (Prasad et al., 2005; Zhu et al., 2019).