Exon 2, which is common to all known isoforms, was disrupted by the in frame insertion of beta-geo and floxed neo. Full length protein was undetected by Western blot analysis of homozygous mutant bone marrow, spleen, and thymus, though truncated polypeptides, translated from transcripts with aberrant splicing from exon 1 to 3, were identified in the spleen and thymus. Low levels of protein arising from the targeted locus were detected by immunohistochemical analysis of homozygous mutant splenocytes.