3.7 kb of sequence in the intergenic region between Il4 and Kif3a was replaced with a loxP site flanked neomycin resistance gene cassette. The deleted region contains DNAse I hypersensitivity sites V and Va. These sites contain enhancers for Il4 transcription. The neo cassette was excised in vitro via the expression of cre recombinase. T lymphocytes and mast cells from homozygous mutant mice express severely reduced Il4 mRNA and protein.