An ENU-induced G to A transition at the first nucleotide of the fifth intron disrupts the mRNA splice donor sequence resulting in production of two aberrantly spliced transcripts that shift the translation reading frame and lead to premature termination codons truncating the protein either immediately before or after the first of the three transmembrane domains. Immunoblot analysis of activated T-cell lysates from homozygous mutant mice using a monoclonal antibody to an N-terminal peptide of the protein verified the absence of the ~27 kD wild-type protein and detected only trace levels of a ~20 kDa protein corresponding to the larger abnormally spliced mRNA encoding through the first transmembrane segment.