Cre-recombinase under the control of mouse mammary tumor virus long terminal repeat (MMTV) was expressed in the salivary gland and mammary epithelial cells of adult mice, and induced recombination in all tissues.

The Cre recombinase from bacteriophage P1 excises intervening DNA sequences located between two unidirectional lox sites positioned on the same linear DNA segment, leaving one lox site behind. Through insertion of lox sites via homologous recombination into the gene of interest and targeting Cre recombinase expression to a specific cell type using a tissue-specific promoter, it is possible to introduce predetermined deletions into the mammalian genome. To delete genes specifically from mammary gland, transgenic mice were created carrying the Cre gene under the control of the mouse mammary tumor virus (MMTV) long terminal repeat (LTR). In adult MMTV-Cre mice, expression of the transgene was confined to striated ductal cells of the salivary gland and mammary epithelial cells in virgin and lactating mice. In contrast to WAP-Cre, however, Cre expression under control of the MMTV LR resulted in recombination in all tissues.