Each human kidney contains about 1.3 million minute tubules lined by a continuous layer of epithelial cells. Fluid that is forced out of the blood from specialized capillaries enters these tubules at their origins and exits into the ureter leading to the urinary bladder. Molecules and ions that the body needs are resorbed back across the epithelial layer of the tubules into the blood. Resorption depends on two separate routes: a transcellular pathway (through the cell cytoplasm via selective pumps and cotransporters on epithelial cell plasma membranes) and a paracellular pathway (through the intercellular space between cells). The paracellular pathway is regulated by tight junctions, intercellular structures in which the plasma membranes of adjacent epithelial cells come into very close contact (see figure, top). Tight junctions circumscribe the entire apical boundary of each cell and can be visualized by freeze-fracture as linear arrays of membrane proteins (1), which include occludin (2) and members of the claudin protein family (3). On page 103 of this issue, Simon et al.(4) now report that a new member of the claudin protein family, paracellin-1, regulates the paracellular transport of magnesium ions (Mg 2+) in the kidney tubule. This is the first report of a tight junction protein that is involved in paracellular ion conductance.