A cassette containing an IRES-lacZ followed by an FRT flanked neomycin selection cassette was inserted in exon 3. No beta-galactosidase activity was detected in embryos. Exon 3 is spliced out of mutant transcripts and splicing of exon 2 to exon 4 causes a frameshift creating a premature stop codon. No protein is detectable by Western blot analysis. (J:199559)