Increased locomotor activity in open-field assessments has been associated with hyperactivity and habituation deficits.39-41 Habituation deficits in humans have been linked to behavioural problems stemming from hyperactivity disorders which result in a lack of attention.42,43 Hyperactivity and inattention as general frequent behavioural problems have been reported in AP-4 associated HSPs,44 perhaps offering an explanation for the hyperactive behaviour observed in Ap4b1 (−/−) and other models of AP-4 deficiency syndromes. The gene discussed is AP4B1; the disease is AP-4 deficiency syndrome.