Mouse mammary tumor virus (MMTV) expression is associated with hyperplastic alveolar growth and subsequent development of mammary cancers in the mouse. The expression of this virus is also controlled by factors involved in the normal proliferation and differentiation of the mammary epithelium. During pregnancy when the mammary gland undergoes massive proliferation, MMTV expression is increased. Steroid hormones and growth factors that play an important role in the proliferation of mammary gland cells are responsible for the increased MMTV expression. In sarcomatous transformation of mouse mammary epithelial cells, MMTV expression is repressed. This repression is due to negative control of MMTV expression by transforming growth factor-beta (TGF beta). This growth factor is produced in high amounts when mammary epithelial cells progress into the transformed state. The expression of MMTV is therefore under multiple control by steroid hormones and growth factors.