To date, the most promising TRD-associated genes code for the serotonin transporter SLC6A4, presynaptic serotonin autoreceptor 5-HTR1A, catechol-O-methyltransferase (COMT), brain-derived neurotrophic factor (BDNF) and the transcription factor CREB1. Here, COMT is linked to treatment resistant depression.