A single loxP site was inserted into intron 2 and a loxP flanked neomycin and thymidine kinase selection cassette was inserted into intron 3. The selection cassette was removed in ES cells by transient Cre expression prior to the production of chimeric mice, leaving exon 2 flanked by loxP sites in the final allele. Cre-mediated recombination deleted exon 2 in the germline.