Targeting took place in ES cells homozyogus for the cre transgene Tg(Prm-cre)70Og, which expresses Cre recombinase only in haploid spermatids. The Carollia (bat) limb enhancer element was used to replace the endogenous mouse enhancer upstream of exon 1 and a floxed neo cassette was inserted into intron 1. Cre-mediated excision removed the neo cassette upon transmission from male chimeric mice. (J:131364)