FKBP5 and major depressive disorder: Studies have investigated the association between anxiety and depression with polymorphisms in candidate genes involved in the neurotransmitter system and the stress response [19], such as Catechol-O-methyltransferase (COMT) [20,21,22], Serotonin receptor 2A (HTR2A) [23] and FK506-binding protein 51 (FKBP5) [24,25].