SLC9A9 and attention deficit-hyperactivity disorder: In recent years, numerous independent studies have implicated NHE6 and NHE9, the two endosomal subtypes (eNHE) of the Na+/H+ exchanger (NHE) gene superfamily, in multiple neurodevelopmental and neuropsychiatric disorders, including autism, severe X-linked intellectual disability (XLID), epilepsy, addiction and attention deficit hyperactivity disorder (ADHD) (Table 1).