Exons 2 through 4 were replaced with a cassette containing beta-galactosidase and neomycin resistance genes. The deleted region contained two start codons encoding high-molecular weight isoforms and 80kD isoforms in exons 2 and 4, respectively. Transcript was undetected by Northern blot analysis using a carboxy terminal coding sequence probe. An 80kD protein was detected in wild-type but not in homozygous mutant mice via Western blot analysis.