Abbreviations: cDNA- complementary deoxyribonucleic acid; CES-D- Center for Epidemiologic Studies Depression Scale; COMT- catechol-O-methyl transferase; DSM- Diagnostic and Statistical Manual of Mental Disorders; MDD- major depressive disorder; HCY- homocysteine; htSNP- haplotype tagging single nucleotide polymorphism; MADRS- Montgomery-Åsberg Depression Rating Scale; MTHFR- methylenetetrahydrofolate reductase; PHQ- Patient Health Questionnaire; RNA- ribonucleic acid; SNP- single nucleotide polymorphism; SSRI- selective serotonin reuptake inhibitor. The gene discussed is MTHFR; the disease is psychiatric disorder.