Exon 3 was flanked with loxP sites by insertion of a floxed TK-neo cassette and a single loxP site into the upstream and downstream introns, respectively. Exon 3 was removed by transient expression of cre recombinase in correctly targeted ES cells. Absence of gene expression was confirmed by Western blot analysis of mouse embryonic fibroblasts (MEFs) derived from homozygous mutant embryos. Immune complex kinase assays showed a lack of enzyme activity in MEFs from homozygous mutants.