ENDOG and depressive symptom measurement: To this end, the present study explores the possible association between the incidence, onset, severity and treatment of depression and the genotypes or alleles of five SNPs: EXOG c.-188T > G (rs9838614), EXOG c.*627G > A (rs1065800), POLG c.-1370T > A (rs1054875), ENDOG c.-394T > C (rs2977998) and ENDOG c.-220C > T (rs2997922).