A loxP site and an frt flanked neomycin selection cassette were inserted in intron 1 and a second loxP site was placed in intron 2. Chimeric mice were bred to a proprietary CMV-cre mouse line on a C57BL/6 background developed by GenOway. This resulted in the excision of both the neomycin selection cassette and exon 2. No protein was detectable immunologically in tissues. (J:138552)