An Emx2 cDNA was inserted into the Otx2 gene as follows: the Emx2 cDNA was inserted at the start codon of the Otx2 gene followed by a loxP-flanked neomycin selection cassette. Resulting chimeric mice were bred to females expressing Cre in resulting zygotes, and the F1 generation was screened for the removal of the neomycin selection cassette.