The expression of the Na+-K+-Cl– co-transporter 1 (NKCC1), which is responsible for the inward transport of chloride, is not affected, leading to a reduced KCC2/NKCC1 ratio (Gogliotti et al., 2018; Hinz et al., 2019) and likely to intracellular chloride accumulation in RTT brains. Here, SLC12A5 is linked to Rett syndrome.