ZEB1 and posterior polymorphous corneal dystrophy: Finally, given that all genetic subtypes of PPCD reported to date are attributed to dysregulation of EMT/MET regulators (OVOL2, GRHL2 and ZEB1) [3,7,8], it is plausible to hypothesise that factors altering the expression of transcription factors in this pathway may also compensate for reduced ZEB1 levels and thus explain incomplete penetrance in some individuals.