Paracellular transport occurs across intercellular tight junctions of epithelia and is important for reabsorption of ions, drugs and nutrients, particularly in the kidney. Paracellular transport is passive. It occurs outside of cells following concentration gradients and transcellular electrical potentials. Claudins tighten the paracellular cleft for solutes and form paracellular ion pores (Furuse, 2009). It is assumed that the extracellular loops specify these claudin functions. The larger first extracellular loop is critical for determining the paracellular tightness and the selective ion permeability (Krause et al., 2008). The shorter second extracellular loop may cause narrowing of the paracellular cleft and have a holding function between the opposing cell membranes.