MMP9 and major depressive disorder: The robustness of these results was further supported by comparison with the results of the largest prior case-control microarray study of depression, the NESDA study (14); several genes differentially expressed in the NESDA study were likewise differentially expressed in these data, for example, MAPK14 and MMP9. Both of these genes were consistently overexpressed in patients with MDD and both code proteins (MAPK14 and matrix metallopeptidase 9) that were hubs of a network of interactions between immune signaling proteins coded by many of the other differentially expressed genes.