A smaller scale study on 225 patients from the European Group for the Study of Resistant Depression (GSRD) and 12 SNPs from candidate genes (i.e., HTR2A [5-hydroxytryptamine receptor 2A], COMT [catechol-O-methyltransferase], ST8SIA2 [ST8 α-N-acetyl-neuraminide α-2,8-sialyltransferase 2], PPP3CC [protein phosphatase 3 catalytic subunit γ] and BDNF [brain-derived neurotrophic factor]) was performed to investigate the interactions between SNPs and clinical variables, including suicidality, in treatment-resistant depression (with Hamilton Rating Scale for depression [HAM-D] >17). This evidence concerns the gene COMT and major depressive disorder.