A loxP site was inserted upstream of exon 7 and a floxed TK-neo derived cassette was inserted downstream of exon 8 via homologous recombination. Exons 7 and 8 encode critical subelements I, II, and III of the protein kinase catalytical domain. The floxed region containing exons 7 and 8 and the TK-neo cassette was removed by transient expression of cre recombinase in correctly targeted ES cells. Western blot analysis of brain membranes from homozygous mutant animals confirmed the absence of gene expression.