Exon 2 was flanked by loxP sites and a loxP-tk-neo cassette was inserted into the upstream intron via homologous recombination. Transient expression of cre recominase in correctly targeted cells facilitated the removal of exon 2 and the loxP-tk-neo cassette. The mutation deletes the cytoplasmic, tranmembrane, and the majority of the catalytic domains. Enzymatic analysis of liver and spleen extracts from homozygous mutant animals showed no detectable enzyme activity.