by correcting a defective genetic profile in the body's cells. The gene, known as p53, has been found to be missing or defective in more than 50 percent of tumors. The therapy involves making a recombinant version of the missing gene -- one engineered in a lab. Doctors hope eventually they will be able to use the recombinant gene to combat a range of tumors, or masses of cancerous cells. They would do this by directly injecting the gene into tumors close to the skin's surface or delivering it intravenously to ones attacking internal organs. In nature, the gene stops tumor growth, and the hope is that when it is put into a tumor, it will trigger "programmed cell death," killing cancer cells but avoiding healthy ones. In the trials, researchers put the p53 gene into tumors of patients who had liver cancer. The gene was injected into an artery that goes directly to the liver, and biopsies later showed the gene was present and active in the liver. Although the process sounds simple, in reality it is not. To get the gene to the tumor, it needs to be attached to a vector, usually some sort of virus that will transport it to its destination. The 16 patients treated with the recombinant p53 gene have had no problems with toxicity so far, although flu-like symptoms and fever have been associated with the treatment. Researchers have begun to test the effectiveness of the procedure in patients with head and neck cancer, but they say conclusions may be years away.