A 46 bp polynucleotide containing a LoxP site was added 404bp upstream exon 2, and a neomycin resistance gene floxed by two LoxP sites was added 533bp downstream exon 3. Cre-mediated excision of exons 2-3 was achieved upon electroporation of a Cre-recombinase expression vector in the correctly targeted ES clones. Protein was undetected in homozygous mutant mice by Western blot analysis and Immnunohistological staining.