BDNF and major depressive disorder: Allele frequencies of the BDNF gene in all study groups were distributed in accordance with the Hardy-Weinberg equilibrium [5-HT1A-group: X2 = 0.02, p = 0.891, 5-HTT-group HS: X2 = 0.72 p = 0.4, MDD patients X2 = 0.17 p = 0.68).