Based on the above findings, to better understand the role of proteasome genes in the mechanisms underlying the MDD treatment response, we (1) investigated the potential roles of three genes (PSMA7, PSMD9 and PSMD13) previously found to be associated with drug responses,16 (2) performed single-gene re-analysis on the Sequenced Treatment Alternatives to Relieve Depression (STAR*D) data set and (3) analyzed the expression profiles of PBCs and fibroblasts. Here, PSMD9 is linked to major depressive disorder.