Surprisingly, although Val66Met in the BDNF gene was associated with antidepressant response in MDD, and serum/plasma levels of BDNF in patients with MDD increased after antidepressant treatment or electroconvulsive therapy, there was no significant difference in the frequency of polymorphisms in the BDNF gene between patients with MDD and healthy controls. Here, BDNF is linked to major depressive disorder.