SLC6A4 and major depressive disorder: Genetic predisposition in MDD relies on a polygenic trait: approximately 40% to 70% of MDD patients have susceptibility loci mainly located in genes involved in the serotonergic system and in the HPA axis, such as polymorphisms in 5-hydroxytryptamine (5HT; serotonin) transporter (5-HTT) and in tryptophan hydroxylase (TPH) genes (TPH1 and TPH2) (95).