In Rett syndrome, KCC2 levels are altered in patients, neurons derived from patient induced pluripotent stem cells and the MECP2 model [178,188,277,278,279], with suggestions that KCC2 is downstream of MECP2 [280], or reduced MECP2-influenced BDNF signalling [281,282,283] suppresses KCC2 [284], which is reduced in patient neurons derived from induced pluripotent stem cells [278]. Here, SLC12A5 is linked to Rett syndrome.