Histone methyltransferases (e.g., GLP, SUV39H1, G9a) are down-regulated in the nucleus accumbens of susceptible mice exposed to chronic social defeat stress, while these molecules were up-regulated in resilient mice exhibiting low depression-like responses, suggesting that histone methylation may be adaptive in the face of stress and protect against development of depression (Covington et al., 2011). This evidence concerns the gene PRDM9 and major depressive disorder.