MAOA and major depressive disorder: Although high-activity MAOA-uVNTR variants (MAOA-H, genotypes associated with 3.5R or 4R alleles) have been associated with MDD [12], some studies have failed to find such an association [13], and various studies have reported an association between low-activity MAOA-uVNTR variants (MAOA-L, genotypes associated with 3R or 5R alleles) and MDD [9].