A loxP site was inserted upstream of exon 1and a floxed neo cassette was inserted 288 bp downstream of exon 2 via homologous recombination. Cre mediated recombination deleted exons 1and 2, intron 1 and part of intron 2. Immunohistochemical analysis confirmed the absence of wild-type protein expression in the ventral midbrain of homozygous embryos at E11.5. (J:175535)