Adding even further to the difficulty of studying BPD-specific effects, BDNF methylation was also found associated with a broad range of psychiatric symptoms and disorders other than BPD, such as bipolar disorder [39, 40], depression [41], schizophrenia [42], and suicidality [43, 44] (reviewed in [45, 46]). This evidence concerns the gene BDNF and major depressive disorder.