Dr. Pablo Tebas, a professor at the University of Pennsylvania who treated the man, said, “It is only one individual, but it is a remarkable result.” Some outside experts were cautious. “At 12 weeks, you can’t say that this therapy works and the patient is controlling it by himself,” said Dr. Jeffrey Laurence, director of the AIDS research laboratory at Weill Cornell Medical College.

Nevertheless, he called the results “amazing.”

The gene therapy did not work so well for five other patients, according to results presented in September at the Interscience Conference on Antimicrobial Agents and Chemotherapy.

Researchers hypothesize that the Trenton patient did better because he had an inherited mutation in one of his two CCR5 genes, making the job easier for the gene therapy. Up to 13.5 percent of his CD4 cells, the main immune cells infected by H.I.V., were missing both copies of the CCR5 gene after the treatment. That is about twice as much as observed in the other patients.

Still, a vast majority of his CD4 cells were not genetically altered and remained susceptible to infection, making it puzzling that the therapy worked at all.

Some scientists said this suggested that freeing as little as 10 percent of CD4 cells from infection might somehow allow the immune system to control the virus. Researchers are contemplating how to increase the percentage of CCR5-deficient cells in patients who lack the Trenton man’s genetic mutation.

A team from the City of Hope and the University of Southern California, and another team from Calimmune and the University of California, Los Angeles, are working on disabling the CCR5 genes in blood stem cells. That would potentially make the entire immune system permanently resistant to infection, though patients would require a stem cell transplant.

Detractors say a functional cure would not offer much beyond existing drug therapy.

“Any approach that is going to require genetic engineering on a patient-by-patient basis is just utterly unrealistic in terms of the global epidemic,” said Dr. Robert Siliciano, professor of medicine at Johns Hopkins.