The coding sequence for rtTA was inserted into the 5'-UTR along with a floxed neomycin selection cassette. The neo selection cassette was eliminated in founder line 7A3 by crossing properly targeted mice to Tg(MMTV-cre)1Mam transgenic animals. When mice were given doxycycline, transgene expression was restricted to mammary epithelial cells as determined by crosses with two different rtTA reporter strains.