A floxed neo cassette was placed upstream of exon 6 and an additional loxP site was placed downstream of exon 6. PC3 ES cells contain a protamine-cre transgene, Tg(Prm-cre)70Og, which expresses Cre recombinase under control of the male-germline specific protamine promoter. Cre-mediated excision removed the neo cassette upon transmission from male chimeras, leaving exon 6 flanked by loxP sites.