Further cellular changes in the KC cornea that have been identified include increased MMP expression and activity (e.g., MMP-1, MMP-2, MMP-9) [106,107,108], elevated expression of pro-inflammatory mediators (e.g., tumor necrosis factor-α, interleukin-6) [109,110,111], and altered ECM-associated pathways (e.g., hydroxyproline, fibronectin, collagen) [112] in KC. This evidence concerns the gene MMP9 and keratoconus.