Emotional disorders such as depression, generalized anxiety disorder and PTSD in humans have been shown to be associated with altered peripheral BDNF levels (Fernandes et al.2014; Matsuoka et al.2013; Wang et al.2015), and these peripheral levels are in turn postulated to reflect central BDNF signaling which is assumed to influence brain function. This evidence concerns the gene BDNF and major depressive disorder.