MMP1 and keratoconus: Studies have shown that the degradation of stromal collagen in KC is accompanied by the altered expression of cytokines, matrix metalloproteinases (MMPs; MMP-1, -3, -7,- 9, -13) with their inhibitors (tissue inhibitor of metalloproteinases-1 (TIMP-1)) and growth factors (epidermal growth factor (EGF), vascular endothelial growth factor (VEGF) as well as nerve growth factor (NGF)) [21–27].