A targeting scheme was designed to flank exons 3 and 4 of the gene with loxP sites. A loxP-flanked TK-neomycin cassette was first introduced to intron 4, then excised to leave a single loxP site. The same loxP-flanked TK-neomycin cassette was then placed in intron 2 to create a floxed drug selection marker. Cre-mediated recombination removed the neo cassette and left exons 3 and 4 floxed. (J:101977)