Among the therapeutic possibilities currently being explored, those that involve biological control mechanisms seem both promising and attractive. Although it has long been thought that cancer cells are not subject to the same regulatory growth control mechanisms that function in normal cells, there is a substantial body of evidence that they can respond to feedback signals telling them to slow or stop their growth. In addition, it has been determined that a relatively small population of cells within a tumor (cancer "stem" or progenitor cells) are responsible for continued tumor growth and that it is these cells that must be controlled if biological anti-tumor therapy is to be effective.