Low levels of serotonin (5-HT; 5-hydroxytryptamine), tryptophan hydroxylase 2 (tph2) expression and 5-hydroxyindoleacetic acid (5-HIAA), the main metabolite of 5-HT, are found in brain tissue and/or cerebrospinal fluid of RTT patients carrying MECP2 mutations [3]. Here, MECP2 is linked to Rett syndrome.