A key function of the proximal tubule is retrieval of most of the vast quantities of NaCl and water filtered by the kidney. Physiological studies using brush border vesicles and perfused tubules have indicated that a major fraction of Cl(-) reabsorption across the apical membrane of proximal tubule cells occurs via Cl(-)-formate exchange. The molecular identity of the transporter responsible for renal brush border Cl(-)-formate exchange has yet to be elucidated. As a strategy to identify one or more anion exchangers responsible for mediating Cl(-) reabsorption in the proximal tubule, we screened the expressed sequence tag database for homologs of pendrin, a transporter previously shown to mediate Cl(-)-formate exchange. We now report the cDNA cloning of CFEX, a mouse pendrin homolog with expression in the kidney by Northern analysis. Sequence analysis indicated that CFEX very likely represents the mouse ortholog of human SLC26A6. Immunolocalization studies detected expression of CFEX, but not pendrin, on the brush border membrane of proximal tubule cells. Functional expression studies in Xenopus oocytes demonstrated that CFEX mediates Cl(-)-formate exchange. Taken together, these observations identify CFEX as a prime candidate to mediate Cl(-)-formate exchange in the proximal tubule and thereby to contribute importantly to renal NaCl reabsorption. Given its wide tissue distribution, CFEX also may contribute to transcellular Cl(-) transport in additional epithelia such as the pancreas and contribute to transmembrane Cl(-) transport in nonepithelial tissues such as the heart.