The microsomal Na+/(PO4)3- transporter isoform 1 (SLC17A3, NPT4 isoform 1) is a member of the anion-cation symporter family. It is expressed in liver, kidney and intestine and may function as a cotransporter of sodium (Na+) and phosphate ((PO4)3- or Pi) across the ER membrane (Melis et al. 2004).