BDNF and major depressive disorder: When comparing the average methylation levels of different BDNF regions between the depression and the control group (Fig. 1; Additional file 1: Table S3), we observed significantly decreased methylation of BDNF exon Ia in the depression group (mean difference 0.30%, p = 0.042), as well as 2 individual CpGs (CpG1: mean difference 0.29%, p = 0.040, CpG2: mean difference 0.46%, p = 0.013).