Robert A. Weinberg has made important discoveries in the field of cancer research. Along with colleagues, he produced tumors in healthy mice by transferring individual cancer-causing genes, called oncogenes, to normal cells. These oncogenes were almost indistinguishable from normal genes--in some cases the difference between a normal gene and an oncogene was a single amino acid along the chain. Weinberg used new forms of genetic engineering to isolate genes in the cells of human tumors. He demonstrated that these oncogenes, when introduced into normal mouse cells grown in a laboratory environment, modified the normal cells and made them cancerous. This work was of great importance to cancer research, as it shifted the focus of biomedical research to investigations at the molecular level. Medical researchers had previously thought cancer was caused in several different ways: by chemical carcinogens, tumor viruses, and radiation. Weinberg's work with oncogenes made it apparent...