The PRL-tTA transgene was designed with a 3.2 kb promoter sequence from the rat prolactin (Prl) gene, a tetracycline-regulated transactivator (tTA) gene, and an SV40 intron/polyA sequence. This 5.2 kb transgene was microinjected into one-cell (C57BL/6 x SJL) mouse zygotes. Mice from founder line 6-5 were bred with C57BL/6;SJL wildtype mice to establish the colony.