A beta-galactosidase cassette with a floxed neomycin resistance gene was inserted to replace the 3' half of exon 3 and all of exons 7 and 8. RT-PCR demonstrated that this replacement truncated the coding sequence in exon 6 by introducing TAG stop codons in all three reading frames and created a bicistronic transcript where beta-galactosidase was under control of the endogenous regulatory sequences by an IRES. (J:99796)