A very unusual blood transplant appears to have cured an American man living in Berlin of infection with the AIDS virus, but doctors say the approach is not practical for wide use.

The man, in his 40s, had a blood stem-cell transplant in 2007 to treat leukemia. His donor not only was a good blood match but also had a gene mutation that confers natural resistance to HIV.

Now, three years later, the recipient shows no signs of leukemia or HIV infection, according to a report in the journal Blood.

"It's an interesting proof-of-concept that with pretty extraordinary measures a patient could be cured of HIV," but it is far too risky to become standard therapy even if matched donors could be found, said Dr. Michael Saag of the University of Alabama at Birmingham.

When the Berlin man's case first surfaced two years ago, experts said said the procedure was too expensive and risky to be practical as a cure but that it might lead to more developments.