Exon 3 was flanked by loxP sites by placing a FRT-flanked neo-cassette with a 3' loxP site upstream of exon 3 with an additional loxP site being placed downstream of exon 3. Founders were crossed with ACTB-FLPe mice to remove the neo selection cassette. Expression levels of Myd88 were unaffected by loxP insertion. Cre-recombination is predicted to excise exon 3 and create a null allele. (J:139001)