The various TRE-E2 transgenic lines produced were mated independently to mice that were heterozygous for both the E2 knockout and the LAP-tTA transgene [Tg(tTALap)Bjd/J; Stock 3272; The Jackson Laboratory, Bar Harbor, ME; NMRI × FVB × C57BL/6J background]. Interbreeding of animals that were heterozygous for both transgenes and the knockout resulted in the production of mice with a variety of genotypes including some animals that were homozygous for the knockout and were positive for both transgenes (we refer to this genotype as the "rescue" genotype). If our strategy for rescuing the neonatal lethal phenotype of the knockout were successful, then those homozygous knockout animals that had both transgenes would survive beyond the neonatal period.