Only one study has shown a putative role of proteasomal subunit genes in the susceptibility to MDD and the antidepressant response.16 The authors observed that the proteasome subunit β4 (PSMD4) gene is involved in etiopathogenesis of MDD, although this result was not replicated in the largest genome-wide association study on MDD.17 Alternatively, Wong et al.16 found significant associations of the treatment response with the 26S proteasome non-ATPase subunit 9 (PSMD9), proteasome alpha type 7 subunit (PSMA7) and PSMD13 genes. Here, PSMD9 is linked to major depressive disorder.