MAOA and major depressive disorder: Therefore, the X-linked MAOA gene polymorphism association with MDD, BD, and SHZ is still inconclusive, which is more likely due to the sex- and phenotype-based differences, and also we cannot rule out the potential interactive contribution of the genotype/phenotype (different repeats allele and phenotype) of both VNTRs and the gene/environment to influence the expression of MAOA and its serum-based levels in different ethnicities.