Particularly interesting were the two genes directly linked to MDD, i.e., GRIA4, the protein-coding gene for subunit 4 of the AMPA glutamate receptor involved in glutamate signalling and neuroplasticity [46], which may be implicated in psychiatric disorders, together with the GRIA1 and GRIA2 genes [47]. Here, GRIA2 is linked to major depressive disorder.