Sequence extending from nucleotide position 21 in exon 1 through position 861 in exon 5 was replaced with a cassette containing lacZ and neo via homologous recombination. Transcript was undetected in homozygous mutant mice via RT-PCR analysis of total spleen and thymis RNA. Homozygous mutant mice were shown to lack the endogenous protein via Western blot analysis of splenocytes.