Claudins allow for Mg transport via the paracellular pathway; that is, it mediates the transport of the ion through the tight junctions between cells that form an epithelial cell layer. In particular, Claudin-16 allows the selective reuptake of Mg in the human kidney. Some patients with mutations in the CLDN19 gene also have altered magnesium transport.
The gene Claudin-16 was cloned by Simon et al. (1999), but only after a series of reports described the Mg flux itself with no gene or protein. The expression pattern of the gene was determined by RT-PCR, and was shown to be very tightly confined to a continuous region of the kidney tubule running from the medullary thick descending limb to the distal convoluted tubule. This localisation was consistent with the earlier reports for the location of Mg re-uptake by the kidney. Following the cloning, mutations in the gene were identified in patients with familial hypomagnesaemia with hypercalciuria and nephrocalcinosis, strengthening the links between the gene and the uptake of Mg.