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. This evidence concerns the gene OVOL2 and posterior polymorphous corneal dystrophy.