Cranial irradiation leads to a decrease of BDNF in the hippocampus (Ji et al., 2014; Son et al., 2015a) and studies in murine models of Rett syndrome or depression already have shown that FTY720 treatment increases hippocampal BDNF levels (Deogracias et al., 2012; di Nuzzo et al., 2015). This evidence concerns the gene BDNF and major depressive disorder.