SLC9A6 and Down syndrome: These include Rett syndrome (MECP2, OMIM#312750), early infantile epileptic encephalopathy (CDKL5, OMIM#300672), FOXG1 syndrome (OMIM#613454), Christianson syndrome (SLC9A6, OMIM#300243), or Pitt-Hopkins syndrome (TCF4, OMIM#610954) (McKnight et al., 2022).