There are two transporters of this type; The genes SLC20A1 and SLC20A2 encode for phosphate transporters 1 and 2 (PiT1 and PiT2 respectively). They both have a broad tissue distribution and may play a general housekeeping role in phosphate transport such as absorbing phosphate from interstitial fluid and in extracellular matrix and cartilage calcification as well as in vascular calcification. These proteins were originally described as retroviral receptors for the gibbin ape leukemia virus receptor 1 (GLVR1, now called PiT1) (O'Hara et al. 1990) and GLVR2 (now called PiT2) (van Zeijl et al.1994). However, they were found to possess Na+-coupled phosphate cotransporter function (Fernandes et al. 1999). The transport is electrogenic with a stoichiometry of 2:1 (Na+:Pi).