A single loxP site was inserted upstream of exon 1 and and additional loxP-flanked nemoycin/thymidine kinase selection cassette was inserted into intron 2. Transient cre expression in ES cells resulted in the deletion of all intervening sequences, including exons 1-2 and the selection cassette. Heterozygous mice exhibit a decrease in protein activity in livers and mammary gland.