Conclusions: The decrease in plasma neopterin concentration was related to DRD with GTPCH deficiency. It may be a useful diagnostic classification of DRD. Background: Monoamine oxidase A (MAOA) deficiency, a defect in biogenic amine neurotransmitter catabolism, was first described in 1993 in a large Dutch family with many borderline mentally retarded males with prominent aggressive behaviour disturbance. So far no new patients with MAOA deficiency were found. Methods: SNP-array analysis for genetic diagnostics and HPLC analysis for urinary metabolite analyses. Results: SNP-array analysis was performed on a young Swedish boy with ADHD-like behaviour, borderline mental retardation and gross obesity. A 1 MB deletion on the X-chromosome was found, encompassing the first three exons of the MAOA gene and no other known genes.