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. Soluble protein cargo enters the nucleus using transport factors that bind to phenylcelanine-glycine-rich domains. At least some integral membrane proteins destined for the nuclear inner membrane enter from the outer membrane via ~120 aa residue-long intrinsically disordered linkers in proteins that also bear a nuclear localization signal. Possibly, an unfolded linker slices through the NPC scaffold to enable binding between the transport factor and the FG domains in the center of the NPC (Meinema et al., 2011).