BDNF and major depressive disorder: This may be because BDNF deficiency in the regions/cells that BDNF promoter IV controls (for example, dorsal regions, hippocampal CA1/prefrontal layer V cells), rather than in the whole brain (BDNF +/-) or in the conditional regions/cells controlled by exogenous promoters (for example, enolase, CaMKII and GFAP), leads to depression-like behavior (detailed discussions in Sakata et al.10).