SLC9A6 and microcephaly: Mutations in NHE6 result in Christianson syndrome (CS), a monogenic disorder causing severe X-linked intellectual disability (XLID), non-verbalism, epilepsy, truncal ataxia, sleep disturbances, postnatal microcephaly, and autistic traits (Christianson et al., 1999; Gilfillan et al., 2008; Tarpey et al., 2009; Garbern et al., 2010; Schroer et al., 2010; Mignot et al., 2013; Ilie et al., 2019; Gruber et al., 2022).