BDNF and bipolar II disorder: Their follow-up study found higher BDNF exon I promoter methylation levels in both patients with MDD and patients with bipolar II disorder, but not in patients with bipolar I disorder; when analysis was performed based on the mood state instead of diagnosis, the study also found that methylation levels of patients in a depressive state were significantly higher, compared to the levels of patients in manic/mixed states [46].