BDNF and bipolar disorder: Many previous studies have identified associations between brain‐derived neurotrophic factor (BDNF) rs6265 (Val66Met) polymorphism and depression disorders (Cardoner et al., 2013), bipolar disorders (Gonzalez‐Castro et al., 2015), and schizophrenia (Kheirollahi et al., 2016), although the findings are not always consistent (Otowa et al., 2009).