 Claudin family proteins form charge-selective paracellular channels in the gastrointestinal tract, and CLDN23 strengthens the epithelial barrier by associating with CLDN3 and CLDN4 to regulate their distribution in tight junctions. This article was authored by Arturo Rias Sandino, Kristen M. Lazardo Soto, Nandini Rajagopal, and others.