VSX1 and posterior polymorphous corneal dystrophy: Currently, three genetically distinct subtypes have been identified: The gene(s) responsible for the PPCD1 subtype is OVOL2. PPCD3 is due to mutations in ZEB1. PPCD4 is linked to the gene GRHL2, and accounts for about 30% of individuals with PPCD and often is associated with corneal steepening (Davidson et al., 2020).