extra integrated MMTV proviruses. These leukemias are active when cells are transferred to other mice. When the virus genome is inserted inside the host genome it is then able to transcribe its own viral genes. In F. U. Reuss and J. M. Coffin (2000) experiments it is mentioned that the expression of the virus genome is activated by an enhancer element that is present in the U3 region of the long terminal repeat of the genome. In addition the expression of the genome is activated specifically in the mammary gland cells. Estrogen is able to further activate the expression of the viral genome. The expression of sag gene which is present in the provirus is responsible for the production of a superantigen. MMTV can be transferred either through an exogenous or endogenous route. If the virus is transferred exogenously, it is passed from the mother mouse to her pups through her milk. Alternatively, pups can be infected vertically through endogenous infection, inheriting the virus directly from their mother in the germline. Mice that become infected in this way have higher rates of occurrence of tumors. A retrovirus is endogenous to its host once the proviral DNA is inserted into the chromosomal DNA. As a result, mice with endogenous MMTV have the virus’s DNA in every cell of its body, as the virus is present in the DNA of the sperm or egg cell from which the animal is conceived. Hormonal responsiveness of integrated MMTV DNA Endogenous MMTV reacts to the whole range of hormones that regulate normal mammary development and lactation, response has been demonstrated to steroid hormones (androgens, glucocorticoids and progestins), as well as prolactin. When the mouse reaches puberty the virus begins to express its messenger RNA in the estrogen sensitive tissues. As a result, after puberty all mammary cells will contain the active retrovirus and begin to replicate in the genome and express viral messenger RNA in all new mammary tissue cells. The MMTV promoter in models of human breast cancer The LTR (long terminal repeat) of MMTV contains a glucocorticoid hormone response element. This glucocorticoid element is a promoter that is often used to construct mice which develop a breast cancer-like disease, because an animal model system for breast cancer close to the human disease is very much looked for. The MMTV promoter is used in the PyMT model system of mouse models of breast cancer