A common functional polymorphism in the BDNF gene (termed the Val66Met polymorphism) has been found to interact with stress exposure to affect risk of depression (Hosang et al., 2014), bipolar disorders (Hosang et al., 2010), schizophrenia (Alemany et al., 2011), or suicide (Pregelj et al., 2011). This evidence concerns the gene BDNF and bipolar disorder.