MMP1 and keratoconus: The tears of patients with keratoconus show, inter alia, an increased expression of metalloproteinase 1 (MMP1) and a decreased tissue metalloproteinase inhibitor (TIMP-1) [57], confirming “the cascade hypothesis of keratoconus”, in which altered levels of individual enzymes lead to cell apoptosis and secondary fibrosis [58].