DRD4 and major depressive disorder: A statistically significant difference was shown in the frequency of DRD4 exon 3 (Ex3) alleles between all patients diagnosed with PUD MDD and the controls, but the frequency of l alleles was lower in the PUD MDD group vs. controls (Table 3 (l 0.18 vs. 0.25, χ2 = 3.99, p = 0.046)).