The following sequences were inserted in the 3' UTR: an IRES, the human diphtheria toxin receptor (HBEGF) cDNA coding region in-frame with the EGFP gene coding region, a loxP site, the mouse Ace (angiotensin I converting enzyme (peptidyl-dipeptidase A) 1) promoter, the cre recombinase gene, a neomycin resistance gene cassette, and a second loxP site. Cre-mediated self-excision removed the cre+neo cassette in the male germline.