SLC12A5 and fragile X syndrome: Importantly, KCC2 dysfunction and Cl− homeostasis disruption are implicated in neurodevelopmental disorders, including Down syndrome (Deidda et al., 2015), fragile X syndrome (He et al., 2014), Rett syndrome (Duarte et al., 2013; Tang et al., 2016) and SCZ (Arion and Lewis, 2011).