A neomycin/thymidine kinase cassette flanked by single loxP sites was inserted into intron 5 and a single loxP site was inserted into intron 6 of the gene. Correctly targeted ES cells were treated with cre recombinase and, in resultant clones, the sequence between the outermost loxP sites (including the neomycin/thymidine kinase cassette, part of intron 5, exon 6, and part of intron 6) was deleted. (J:58700)