A year ago, when chemotherapy stopped working against his leukemia, William Ludwig signed up to be the first patient treated in a bold experiment at the University of Pennsylvania. Mr. Ludwig, then 65, a retired corrections officer from Bridgeton, N.J., felt his life draining away and thought he had nothing to lose.

Doctors removed a billion of his T-cells — a type of white blood cell that fights viruses and tumors — and gave them new genes that would program the cells to attack his cancer. Then the altered cells were dripped back into Mr. Ludwig’s veins.

At first, nothing happened. But after 10 days, hell broke loose in his hospital room. He began shaking with chills. His temperature shot up. His blood pressure shot down. He became so ill that doctors moved him into intensive care and warned that he might die. His family gathered at the hospital, fearing the worst.

A few weeks later, the fevers were gone. And so was the leukemia.

There was no trace of it anywhere — no leukemic cells in his blood or bone marrow, no more bulging lymph nodes on his CT scan. His doctors calculated that the treatment had killed off two pounds of cancer cells.

A year later, Mr. Ludwig is still in complete remission. Before, there were days when he could barely get out of bed; now, he plays golf and does yard work.

“I have my life back,” he said.

Mr. Ludwig’s doctors have not claimed that he is cured — it is too soon to tell — nor have they declared victory over leukemia on the basis of this experiment, which involved only three patients. The research, they say, has far to go; the treatment is still experimental, not available outside of studies.

But scientists say the treatment that helped Mr. Ludwig, described recently in The New England Journal of Medicine and Science Translational Medicine, may signify a turning point in the long struggle to develop effective gene therapies against cancer. And not just for leukemia patients: other cancers may also be vulnerable to this novel approach — which employs a disabled form of H.I.V.-1, the virus that causes AIDS, to carry cancer-fighting genes into the patients’ T-cells. In essence, the team is using gene therapy to accomplish something that researchers have hoped to do for decades: train a person’s own immune system to kill cancer cells.

Two other patients have undergone the experimental treatment. One had a partial remission: his disease lessened but did not go away completely. Another had a complete remission. All three had had advanced chronic lymphocytic leukemia and had run out of chemotherapy options. Usually, the only hope for a remission in such cases is a bone-marrow transplant, but these patients were not candidates for it.

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