A single loxP site was inserted into intron 10 and a loxP flanked neomycin selection cassette was inserted into exon 11. The loxP-flanked sequences were deleted in ES cells by transient Cre expression prior to the production of chimeric mice. The final heritable allele contains a single loxP site replacing exon 11 and flanking intronic sequences. Immunohistochemistry experiments confirmed that no detectable protein is expressed from this allele.