The cation-chloride cotransporter family (SLC12 gene family) are membrane proteins that cotranslocate chloride (Cl-) with either Na+, K+, or both cations electroneutrally. The general topology of these proteins feature 12 transmembrane domains flanked by hydrophilic NH2 and COOH-terminal domains. They are secondary transporters and movement of these cations is determined by gradients established by primary transporters such as Na+-K+-ATPase. Cotransporters that use Na+ as the driving force move Cl- into the cell because Na+ concentration is higher in the extracellular region. Conversely, cotransporters that use K+ as the driving force move Cl- out of the cell because K+ concentration is higher inside the cell.

The SLC12 gene family contains nine members, of which seven are clearly characterized genes and two are orphans. They encode cotransporter proteins which are 1) involved in Cl- homeostasis, 2) regulate cell volume, 3) involved in transepithelial ion movement (salt reabsorption in the kidney) and 4) involved in response to neurotransmitters such as GABA.

Three different cotransporter subtypes are expressed by the seven characterized genes; one thiazide-sensitive Na+/Cl- cotransporter, two loop diuretic-sensitive Na+, K+/2Cl- cotransporters and four K+/Cl- cotransporters (Gamba G, 2005; Hebert SC et al, 2004).