A gene trap vector containing beta-geo, puro, and flanking long terminal repeats inserted into an intron of the endogenous locus, corresponding with human intron 5. RT-PCR analysis of splenic mRNA indicated an absence of normal transcript in homozygous mutant mice. Though mutant transcript species comprising sequence either 5' or 3' to the gene trap insertion were detected, transcript containing an undisrupted catalytic domain was not detected in homozygous mutant mice. Endogenous enzymatic activity was not detected in homozygous lymph node lysates.