CA9 and keratoconus: VHL [12,13], VEGF (vascular endothelial growth factor [14]), CAIX (carbonic anhydrase IX [15]), PTEN [15], CR1 (complement C3b/C4b receptor 1 [16]), MERTK (MER proto-oncogene tyrosine kinase [17]), PIK3CA (phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha [18]), and AKT1 (AKT serine/threonine kinase 1 [19]) are among the most popular KC biomarkers.