A PGK-neomycin cassette replaced most of exon 5, which encodes the extracellular domain. Northern analysis detected a faint, aberrant ~1.3kb transcript in thymocytes of homozygous mutant mice. PCR studies of cDNA synthesized from total thymocyte RNA did not detect wild-type mRNA in homozygous mutant mice using an internal, exon 8 oligo probe. However, these studies did detect a faint truncated transcript in homozygous mutant thymocytes, indicating that exon 5 was skipped in splicing events in the mutant allele. Western blot analysis of homozygous mutant thymocytes using a rabbit antiserum to the cytoplasmic carboxy terminal ten amino acids detected a small amount of a truncated protein.