PAX6 and isolated aniridia: The absolute requirement for the presence of the DRR (or at least some of the enhancers within it) can explain why the aniridia phenotype in ‘position effect’ patients is indistinguishable from aniridia in patients carrying coding region mutations in PAX6 (Crolla and van Heyningen, 2002; van Heyningen and Williamson, 2002).