Mediates electroneutral potassium-chloride cotransport in mature neurons and is required for neuronal Cl(-) homeostasis. As major extruder of intracellular chloride, it establishes the low neuronal Cl(-) levels required for chloride influx after binding of GABA-A and glycine to their receptors, with subsequent hyperpolarization and neuronal inhibition (PubMed:9930699). Involved in the regulation of dendritic spine formation and maturation (PubMed:22345354). Inhibited by WNK3. Homomultimer and heteromultimer with other K-Cl cotransporters. Interacts with AP2A1 (By similarity). Highly expressed in brain. Not detected in other tissues. Highly expressed in pyramidal neurons and in neurons throughout the cortex, hippocampus, the granular layer of the cerebellum and in groups of neurons throughout the brainstem. Barely detectable in dorsal-root ganglions. Detected in thalamus, but not in hippocampus and neocortex at E20. At birth barely detectable in hippocampus. Expression increases steeply from day 5 to day 9 and then stabilizes at adult levels. Belongs to the SLC12A transporter family.