Candidate gene studies have identified a number of genes, such as catechol-O-methyltransferase (COMT), brain-derived neurotrophic factor (BDNF), neuregulin-1 (NRG-1), and disrupted in schizophrenia (DISC-1) that appear to be shared risk factors for schizophrenia, bipolar disorder, and MDD (41). This evidence concerns the gene BDNF and bipolar disorder.