Contrasting with the low frequency of lens agenesis, severe forms of retinal coloboma affect all Rara−/−;Rarg−/− KO mutants, while a milder form of coloboma, restricted to the optic disc, is present in a majority of Rarb−/−;Rarg−/− KO mutants. The gene discussed is RARG; the disease is Retinal coloboma.