Multiple studies of tear composition have shown increased levels of MMP-9 protein in KC, including one patient with asymmetrical KC in which MMP-9 was upregulated only in the tears of the affected eye.168, 169, 170This increase in MMP-9 was confirmed with an accompanying upregulation of MMP-9 mRNA in the corneal epithelium.170. The gene discussed is MMP9; the disease is keratoconus.