Exon 6, which is excised in some isoforms, was deleted and replaced with a neomycin selection cassette. Immunofluorescence analysis, using an antibody raised against all isoforms, of cells isolated from homozygous mutant mice showed an absence of encoded protein in B lymphocytes and low levels of expression in peripheral T cells. Flow cytometric analysis of peripheral T cells confirmed that protein containing the region encoded by exon 6 was absent, while protein containing the region encoded by exon 5 was present at low levels. (J:13163)