The notion that ST3GAL3 inactivation causes severe intellectual disability with distinguishable developmental, neurological and neuropsychiatric phenotypes led us to predict that more subtle allelic variation of ST3GAL3 expression may increase the vulnerability to ADHD and related conditions, or predispose individuals to the effects of other genetic and/or environmental risk factors. This evidence concerns the gene ST3GAL3 and attention deficit-hyperactivity disorder.