For example, the apolipoprotein (Apo) ε2 allele is a potential confounding covariate for the MAOA uVNTR polymorphism in MDD: the high activity MAOA uVNTR alleles are associated with MDD when the Apo ε2 allele, which is a protective factor for MDD, is adjusted for [19]. This evidence concerns the gene AOPEP and major depressive disorder.