UT-A1, a kidney-specific urea transporter is expressed in the renal collecting duct where it mediates trans-epithelial urea transport and is a target for regulation by vasopressin. Urea movement out of the collecting duct in the inner medulla of the kidney allows accumulation of urea in the medullary interstitium, thereby allowing maximum water reabsorption from the collecting ducts. (The antibody also recognizes a second protein from the UT-A gene driven by transcription from an alternative promoter and expressed in the thin descending limb of Henle, viz. UT-A2.) (1, 2)