Two common serotonin transporter (SERT) untranslated region gene variants have been intensively studied, but remain inconclusively linked to depression and other neuropsychiatric disorders. We now report an uncommon coding region SERT mutation, Ile425Val, in two unrelated families with OCD and other serotonin-related disorders. Six of the seven family members with this mutation had OCD (n= 5) or obsessive-compulsive personality disorder (n= 1) and some also met diagnostic criteria for multiple other disorders (Asperger's syndrome, social phobia, anorexia nervosa, tic disorder and alcohol and other substance abuse/dependence). The four most clinically affected individuals—the two probands and their two slbs—had the I425V SERT gene gain-of-function mutation and were also homozygous for 5′-UTR SERT gene variant with greater transcriptional efficacy.