BDNF and major depressive disorder: In depression-related models, reduced acetylation at sites such as H3K9ac and H4K12ac has been associated with decreased expression of key mood-regulating genes, including brain-derived neurotrophic factor (BDNF) and the serotonin transporter (SLC6A4) (Covington et al., 2011; Schroeder et al., 2007).