Aniridia (iris hypoplasia) is an autosomal dominant congenital disorder of the eye. Mutations in the human aniridia (PAX6) gene have now been identified in many patients from various ethnic groups. In the study reported here we describe PAX6 mutations in one sporadic and five familial cases with aniridia. Of the four different mutations identified, one was identical to a previously reported mutation (C-->T transition at codon 240), and three were novel: two in the glycine-rich region and one in the proline/serine/threonine-rich (PST) region. One PAX6 mutation found in the PST region was associated with cataracts in an aniridia family. Another splice mutation in the PST domain occurred in an aniridia patient with anosmia (inability to smell). The six new aniridia cases reported here have mutations predicted to generate incomplete PAX6 proteins. These results support the theory that human aniridia is caused by haploinsufficiency of PAX6.