A loxP-flanked neomycin cassette followed by an internal ribosomal entry site (IRES) and an EGFP gene was inserted into sequences corresponding to the 3' untranslated region in exon 4. The neomycin cassette was removed by Cre mediated germline recombination in chimeric mice to generate the final allele. These mice express green fluorescent protein under the control of the Il4 promoter.