MAOA and major depressive disorder: On the other hand, we observed that the rare repeat alleles (MAOA-RA = 4.5R, 5.5R, 6R) were found to have a significant association with higher expression of MAOA in all phenotypes (BD, MDD, and SHZ) in comparison to our control group; however, the serum MAOA levels in rare repeat allele carriers in controls were also found to be lower than MAOA-HA allele carriers within the control group.