MAOA and major depressive disorder: The G allele of the MAOA gene polymorphism (rs6323) encodes the higher activity form of the enzyme and thereby decreases dopaminergic tone, and is significantly associated with a poor antidepressant response in MDD patients (Shih et al., 1999; Ducci et al., 2006; Leuchter et al., 2009; Xu et al., 2011).