Four SLC17 genes are thought to encode type I Na+-dependent phosphate co-transporters in humans. SLC17A1 (NPT1) encodes Na+-dependent phosphate co-transporter 1 (Na/Pi-4). It is abundant in human kidney cortex, liver and brain and is important for the resorption of phosphate by the kidney. It does this by actively transporting phosphate into cells via Na+ cotransport in the renal brush border membrane (Chong SS et al, 1993).

Three close relatives of NPT1 have been identified through genomic analysis and designated NPT3 (SLC17A2), NPT4 (SLC17A3) and a putative small intestine sodium-dependent phosphate co-transporter (SLC17A4). None of these three proteins have been functionally characterized yet.