Developing tissues are sensitive to Pax6 dosage: heterozygous human mutations in Pax6 result in aniridia and forebrain abnormalities [24],[25], as do a number of mouse mutations (for reviews, see [7],[26]), humans and mice homozygous for Pax6 mutations typically lack eyes and have marked microcephaly and absent olfactory bulbs [5],[6],[19], whereas increasing Pax6 levels in transgenic mice results in microphthalmia and forebrain abnormalities [27]–[29]. The gene discussed is PAX6; the disease is isolated aniridia.