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. Involved in the regulation of dendritic spine formation and maturation. Inhibited by WNK3. Homomultimer and heteromultimer with other K-Cl cotransporters (By similarity). Interacts with AP2A1. Detected on dendrites, but not on axons of spinal cord neurons and at GPHN-positive inhibitory synapses. Isoform 2 expressed in brainstem and spinal cord, isoform 1 expressed in brainstem, spinal cord and olfactory bulb of 17 dpc embryos. Isoforms 1 and 2 expressed in all parts of the brain and spinal cord in postnatal day 14 mice. Isoform 1 predominant isoform in 17 dpc brain. Isoform 2 predominant isoform during postnatal development. Detected in the ventral horns of the spinal cord at 12.5 dpc, and throughout the spinal cord at birth. Death at birth due to severe motor deficits including respiratory failure. Mice lacking isoform 2 die within 2 weeks after birth. Belongs to the SLC12A transporter family. Truncated N-terminus.