A targeting vector to insert a loxP site upstream of exon 3, containing the translation initiation sequence, and a second loxP site downstream of exon 4 of the interleukin 1 receptor, type I (Il1r1) gene. An FRT-flanked neomycin resistance (neo) cassette was placed downstream of the 3' loxP site. Flp-mediated recombination removed the FRT-flanked neo cassette.