The expression of BDNF was downregulated by the overexpression of miR-34a, miR-30a-5p, let-7d, miR-10a-5p, miR-375 and miR-155 in neurons or brain tissues [26–31], and the low expression levels of BDNF in brain tissue were thought to be part of the main pathogenesis of MDD [32]. This evidence concerns the gene BDNF and major depressive disorder.