VSX1 and posterior polymorphous corneal dystrophy: We will focus on two main candidate genes, visual system homeobox 1 (VSX1) and superoxide dismutase 1 (SOD1). VSX1 is located within a linkage locus for a corneal dystrophy called posterior polymorphous dystrophy (PPCD) [224–226], which has been associated with KC [227–233].