This transgene directs the expression of Cre recombinase under the control of the mouse mammary tumor virus (MMTV) long terminal repeat. The viral MMTV promoter directed expression of Cre recombinase in the secretory epithelium of the mammary gland, the salivary gland, seminal vesicle, skin, erythrocytes, B and T cells. This line does not express cre in oocytes.