Aniridia is a congenital malformation of the eye, chiefly characterised by iris hypoplasia, which can cause blindness. The PAX6 gene was isolated as a candidate aniridia gene by positional cloning from the smallest region of overlap of aniridia-associated deletions. Subsequently PAX6 intragenic mutations were demonstrated in Smalleye, a mouse mutant which is an animal model for aniridia, and six human aniridia patients. In this paper we describe four additional PAX6 point mutations in aniridia patients, both sporadic and familial. These mutations highlight regions of the gene which are essential for normal PAX6 function. In addition, the frequency at which we have found PAX6 mutations suggests that lesions in PAX6 will account for most cases of aniridia.