Cancer in its various forms continues to be a major U.S. health problem, accounting for 550,000 deaths a year, as well as much disability and suffering. Treatment for cancer has traditionally consisted of three modalities: surgery, radiation therapy, and chemotherapy. Advances with all three modalities over the years have produced long-term remissions and/or cures in certain types of cancer such as the leukemias, and prolonged survival for many other patients. Much remains to be accomplished, however, especially with respect to the treatment of solid tumors, including some of the most common cancers such as those of the lung, colon, breast, ovary, prostate and kidney. New types of less toxic and debilitating therapy are needed.