The startling case of an AIDS patient who underwent a bone marrow transplant to treat leukemia is stirring new hope that gene-therapy strategies on the far edges of AIDS research might someday cure the disease.

The patient, a 42-year-old American living in Berlin, is still recovering from his leukemia therapy, but he appears to have won his battle with AIDS. Doctors have not been able to detect the virus in his blood for more than 600 days, despite his having ceased all conventional AIDS medication. Normally when a patient stops taking AIDS drugs, the virus stampedes through the body within weeks, or days.

“I was very surprised,” said the doctor, Gero Hütter.

The breakthrough appears to be that Dr. Hütter, a soft-spoken hematologist who isn’t an AIDS specialist, deliberately replaced the patient’s bone marrow cells with those from a donor who has a naturally occurring genetic mutation that renders his cells immune to almost all strains of HIV, the virus that causes AIDS.

The development suggests a potential new therapeutic avenue and comes as the search for a cure has adopted new urgency. Many fear that current AIDS drugs aren’t sustainable. Known as antiretrovirals, the medications prevent the virus from replicating but must be taken every day for life and are expensive for poor countries where the disease runs rampant. Last year, AIDS killed two million people; 2.7 million more contracted the virus, so treatment costs will keep ballooning.

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.

Back in 1996, when “cocktails” of antiretroviral drugs were proved effective, some researchers proposed that all cells harboring HIV might eventually die off, leading to eradication of HIV from the body — in short, a cure. Those hopes foundered on the discovery that HIV, which integrates itself into a patient’s own DNA, hides in so-called “sanctuary cells,” where it lies dormant yet remains capable of reigniting an infection.

But that same year, researchers discovered that some gay men astonishingly remained uninfected despite engaging in very risky sex with as many as hundreds of partners. These men had inherited a mutation from both their parents that made them virtually immune to HIV.

Read the full story from the Wall Street Journal.