This study examines the feasibility of using gene therapy to prevent some of the toxicities of an intensive chemotherapy regimen in patients with metastatic breast cancer. Patients who do not wish to participate in the gene therapy procedures will be offered identical chemotherapy on a different protocol. Patients will be treated initially with chemotherapy which is active against breast cancer, but which has a low potential to hurt blood-forming cells. Then, the patient will receive high dose chemotherapy, during which time blood cells which are capable of rebuilding patients' bone marrows will be removed from the patients' bloodstream. We will use these blood cell collections to isolate peripheral blood progenitor cells (PBPCs), those cells which are thought to be the forbears of all other blood cells.