In subantimicrobial doses it is also effective as primary treatment for rosacea and sterile corneal ulceration.36,37 Previous studies using experimental dry eye models demonstrated that doxycycline was efficacious in decreasing gelatinolytic activity in the ocular surface epithelia, as well as decreasing levels of MMP-9 mRNA transcripts, and preventing experimental dry eye-induced increase in IL-1 and TNF-α.38 Doxycycline also improves corneal surface regularity and barrier function.39 The gene discussed is MMP9; the disease is Keratoconjunctivitis sicca.