An FRT-flanked selection vector containing a neomycin resistance gene and a single 5' loxP site was inserted upstream of exon 3 and a second loxP site was inserted downstream of exon 3. The selection cassette and exon 3 were subsequently removed from properly targeted mice by crossing to transgenic mice carrying Tg(EIIa-cre)C5379Lmgd. (J:94077)