More importantly, regulation of CNS development by Pax6 is sensitive to Pax6 dosages that is evidenced by the followings: 1) heterozygous human mutation of Pax6 results in aniridia and forebrain abnormalities; 2) homozygous human and mouse mutations of Pax6 lack eyes and have marked microcephaly and absent olfactory bulbs; 3) increased expression of Pax6 in transgenic mice results in microphthalmia and forebrain abnormalities [10]. This evidence concerns the gene PAX6 and microcephaly.