A loxP site was introduced with a neo-TK cassette into exon 3 and a second loxP site was introduced with a TK-Hph cassette into exon 5 via homologous recombination. Transient expression of cre recombinase in correctly targeted ES cells removed floxed regions containing exons 3-5 and the 2 TK genes. The deleted exons encode complement control protein (CCP) modules 2 and 3. RT-PCR analysis detected a truncated mRNA in homozygous mutant animals lacking exons 3-5. Functional assays confirmed the mutant protein was devoid of protein activity.