Renal tubular reabsorption of phosphate is critical to the maintenance of phosphate homeostasis in mammals, and the brush-border membrane Na-P(i) cotransport systems in proximal tubules play a major role in this process. We have isolated a cDNA encoding a mouse sodium-dependent phosphate transport protein (Npt1), which is expressed primarily in the kidney. This protein is highly similar to its human and rabbit homologues, based on nucleotide and amino acid comparisons. The presence of potential Asn-linked glycosylation and protein kinase C phosphorylation sites that are conserved among all three homologues suggests that these sites may be important in the function and regulation of this protein. The Npt1 gene was mapped to mouse chromosome 13, close to the Tcrg locus. By both in situ hybridization and reverse transcription-polymerase chain reaction, Npt1 mRNA was localized predominantly to the proximal tubule.