In line with its pivotal role in regulating inhibitory and excitatory neurotransmission, alterations in KCC2 expression and function have emerged as a common mechanism underlying a range of human brain disorders, including epilepsy (Puskarjov et al., 2014), schizophrenia (Hyde et al., 2011), Rett syndrome (Banerjee et al., 2016; Hinz et al., 2019), and autism spectrum disorders (ASD) (Merner et al., 2015). Here, SLC12A5 is linked to Rett syndrome.