The MMTV-rtTA-SV40 transgene was designed with the 1.5 kb mouse mammary tumor virus long terminal repeat (MMTV-LTR) promoter, the reverse tetracycline transactivator gene and VP16 activation domain (rtTA-VP16), and SV40 intron/polyA sequence. The rtTA-VP16-polyA sequence is inserted into exon 3 of the rabbit beta-globin gene immediately following the MMTV promoter. This 3.75 kb transgene was injected into one-cell (C57BL/6 x SJL) mouse zygotes. Mice from founder line 4-1 were bred with C57BL/6;SJL wildtype mice to establish the colony. (J:134284)