Exons 1 and 2 encoding amino acids 1-203, which includes the translation start codon, degenerate zinc finger 1, and zinc fingers 2 and 3 of the DNA binding domain, were replaced with a neomycin resistance gene via homologous recombination. Homozygous mutant embryos were observed by genotype analysis at E6.5-E7.5. (J:72861)