POLG and major depressive disorder: The results presented in the current paper indicate that SNPs located in genes encoding (i) POLG, a subunit of Pol γ involved in the replication, repair and degradation of mtDNA; (ii) ENDOG, an endonuclease with ability to induce apoptosis and autophagy as well as one of the candidates that is speculated to degrade damages mtDNA; and (iii) EXOG, an enzyme involved in mtBER are associated with the occurrence, onset, treatment and severity of depression (Figure 5).