A targeting vector was designed to place a loxP site just upstream of the ATG translational start site and a loxP-flanked PGK-Neo cassette downstream of the same exon of the targeted gene. The construct was injected into RW-4 ES cells. Chimeric mice were bred to C57BL/6 mice, and the neomycin selection cassette was excised from these mice by crossing to FVB transgenic mice with widespread Cre recombinase expression. (J:95410)