Cancer is the second leading cause of death in the United States. About 1,200,000 new cases will be diagnosed and over 550,000 Americans are expected to die of cancer in 2001. Despite developments in surgery, chemotherapy, and radiation therapy of cancer, new strategies for treatment are sorely needed. Advances in knowledge of the molecular basis for cancer have led to the understanding that cancer is a multistep process involving a variety of gene alterations. These genetic lesions accumulate until malignant cells are created and proliferate to form a malignant neoplasm, or tumor. To take advantage of these new understandings, gene therapy has emerged as a new treatment strategy. It involves introducing genes to either reverse or counteract the molecular defects or alterations associated with cancer cells. The potential advantage of gene therapy is that it can be directed to tumor cells with greater specificity than conventional chemotherapy. Thus it is less likely to affect normal cells or to cause undesired systemic effects.