MMP9 and Keratoconjunctivitis sicca: Previous studies have shown that mRNA expression levels of interleukin (IL)-1β, IL-6, IL-8, tumor necrosis factor (TNF)-α, interferon (IFN)-γ, and matrix metallopeptidase (MMP)-9 were significantly increased in tears and conjunctiva of dry eye patients compared to normal controls [16,17,18,19].