MMP9 and major depressive disorder: Our analysis also revealed that members of the extracellular proteolytic system, composed of matrix metalloproteinases (MMPs) and their endogenous tissue inhibitors (TIMPs), were modulated in the two disease groups (increased MMP-9 levels, and, to a lesser extent, decreased MMP-2 levels in MDD; increased TIMP-1 levels and MMP-9 levels in schizophrenia).