The endogenous locus was modified by insertion into the first intron of a construct containing the rabbit beta-globin 3' splice site followed by the cre/ERT2 fusion gene, which encodes Cre recombinase joined to a mutant hormone-binding domain of the human estrogen receptor that is activated by 4-hydroxytamoxifen (4-OHT), but not by the natural ER ligand 17-beta-estradiol. The fusion gene is followed by the rabbit beta-globin polyadenylation signal and a PGK-puro (puromycin resistance) cassette. Western blot analysis confirmed expression of the cre/ERT2 fusion product.