The coding sequence (exons 4 through 13) was replaced with a loxP site, cDNA encoding an alternatively spliced isoform for Pax6 that contains 14 amino acid inserted within the PD domain that changed its DNA binding specificity (5a), an FRT-flanked neomycin resistance/STOP cassette, loxP site and cDNA encoding Pax2. This allele expressed the 5a isoform of Pax6 (Pax6_i5a).