COL8A2 and posterior polymorphous corneal dystrophy: Locus heterogeneity has been identified for PPCD with mutations in the COL8A2 (collagen type VIII alpha 2 chain) gene on chromosome 1p34.3 (PPCD2), the ZEB1 (zinc finger E-box binding homeobox 1) gene on chromosome 10p11.2 (PPCD3), and the GRHL2 (grainyhead like transcription factor 2) gene on chromosome 8q22.3 (PPCD4) [6,7,8,9].