DNA methylation normally reduces gene expression, and DNA methyltransferases (DNMTs), such as DNMT1, DNMT2, DNMT3A, DNMT3B, and DNMT3L, can catalyze DNA methylation; DNMT1 and DNMT3A are expressed at high levels in the somatic nuclei of mice exposed to major depression-like behaviors, which may regulate synaptic and structural plasticity as well as memory formation [41]. Here, DNMT3A is linked to major depressive disorder.