BDNF and major depressive disorder: Some clinical researches also evidenced this association between the BDNF Met carrier and lower BDNF levels in the patients with major depressive disorder (Colle et al., 2017), schizophrenia, and bipolar disorder (Aas et al., 2014) respectively, indicating BDNF Met carrier may play a vital role in regulating BDNF protein expression (Youssef et al., 2018).