NPTN and major depressive disorder: Moreover, a literature search performed among these proteins using PubMed and Google Scholar indicated that 9 of them have been previously associated with antidepressant drug response or with MDD in clinical or preclinical studies (8 upregulated: RPN2, HSPA9, NPTN, AP2B1, UQCRC2, RACK-1, TOLLIP, and 1 downregulated protein, TLN2, Table 2).