A carefully selected bone-marrow transplant for a leukemia patient appears to have stopped the patient’s HIV infection: he shows no signs of the virus in his blood nearly two years after the procedure. While it’s difficult to draw any conclusions from a single case, the outcome gives hope for new avenues for AIDS treatment.

Some people are genetically resistant to HIV infection, even when they engage in frequent high-risk behavior–a fact that hematologist Gero Hütter wanted to take advantage of when faced with a 42-year-old patient with both leukemia and HIV. The patient needed a bone-marrow transplant, so Hütter searched compatible blood donors for a specific genetic mutation known to protect against most strains of HIV. Doctors then irradiated the patient’s immune system and transfused the donor cells.

The transplant surgeons halted his HIV drugs to give the new cells time to take root. They planned to resume the drugs once HIV was found in the patient’s blood. But according to an article in the Wall Street Journal, the virus never came back.

Nearly two years later, standard tests haven’t detected virus in his blood, or in the brain and rectal tissues where it often hides … Normally when a patient stops taking AIDS drugs, the virus stampedes through the body within weeks, or days.

The treatment is unlikely to be broadly applicable: only about two-thirds of cancer patients survive the procedure. But scientists may be able to mimic the effect by reengineering patients’ own cells. Doctors are already testing gene-therapy treatments that target the gene that renders some people immune to the virus.

According to the WSJ,

While cautioning that the Berlin case could be a fluke, David Baltimore, who won a Nobel prize for his research on tumor viruses, deemed it “a very good sign” and a virtual “proof of principle” for gene-therapy approaches. Dr. Baltimore and his colleague, University of California at Los Angeles researcher Irvin Chen, have developed a gene therapy strategy against HIV that works in a similar way to the Berlin case. Drs. Baltimore and Chen have formed a private company to develop the therapy.