When Gero Huetter, a German hematologist, performed a bone-marrow transplant on a leukemia patient who was also suffering from AIDS, as a side experiment he transplanted the marrow of a donor who was naturally resistant to HIV, figuring that the new marrow would produce white blood cells resistant to the virus. Twenty months later, Huetter claims that the man shows no signs of AIDS. However many are skeptical whether the patient is really cured. HIV is known to hide in various parts of the body, and though Huetter has tested the patient, doctors wonder if he has been thorough. And even Huetter says that bone-marrow transplants are not a viable cure: They kill about a third of their patients and can’t be justified in treating anything but late-stage leukemia.