The present invention is related to mammary tumor virus (MTV). MTV represents a group of retroviruses which possess very high homology to mouse mammary tumor virus (MMTV), a virus known to cause neoplastic mammary disease in mice. As described herein, MTV's have been identified in human, cat, and Rhesus macaque. The present invention specifically provides for recombinant nucleic acids and polypeptides derived from these MTV's as well as methods for using these biological molecules.