Glutathione S-transferases (GSTs) are proteins found in high concentrations (2% of soluble protein) in the luminar cells of the proximal and distal tubules. Different isoforms are found in different parts of the nephron, therefore, by measuring urinary alpha GST (αGST) one can study the proximal tubule and, by measuring GSTYb1 in rats, one can study the distal tubule. Renal tubular injury can thus be precisely localized. These proteins are not released in the healthy rat and as such act as a very sensitive indicator of site specific injury.