Two notable exceptions are the DRD4 and DAT gene: both have variable tandem repeat polymorphisms which may have a "single gene" influence on susceptibility to MDD.
Patients with mood disorders (N = 125: bipolar subtype, n = 100; major depressive disorder subtype, n = 25) were followed prospectively for an average of 53 months and were typed for DRD2 (Ser311/Cys311: n = 121, VNTR: n = 63), DRD4 (n = 125) and GABRA1 (n = 61) variants using polymerase chain reaction (PCR) techniques.
In the present study, 102 unipolar MDD patients and 172 healthy controls were genotyped for polymorphisms in four serotonergic and three dopaminergic candidate genes [tryptophan hydroxylase (TPH), serotonin receptor 2A (HTR2A), serotonin receptor 2C (HTR2C), serotonin transporter promoter region (5-HTTLPR), dopamine receptor D4 (DRD4), dopamine transporter (DAT1) and catechol-O-methyl transferase (COMT)].