A FRT/loxP flanked neomycin cassette was inserted upstream of exon 4 in intron 3-4 in an opposite direction in relation to the target gene. A single loxP site was inserted downstream of exon 4. Cre-mediated recombination removed both the selection cassette and exon 4. cDNA sequencing revealed that excision of exon 4 leads to a frameshift mutation with concomitant premature stop codon after 59 residues. (J:262620)