A bone marrow transplant using stem cells from a donor with natural genetic resistance to the AIDS virus has left an HIV patient free of infection for nearly two years, German researchers have said.

The patient, an American living in Berlin, was infected with the human immunodeficiency virus that causes AIDS and also had leukemia. The best treatment for the leukemia was a bone marrow transplant, which takes the stem cells from a healthy donor's immune system to replace the patient's cancer-ridden cells.

Dr Gero Hutter and Thomas Schneider of the Clinic for Gastroenterology, Infections and Rheumatology of the Berlin Charite hospital on Wednesday said the team sought a bone marrow donor who had a genetic mutation known to help the body resist the AIDS virus.

The mutation affects a receptor, a cellular doorway, called CCR5 that the AIDS virus uses to get into the cells it infects.

When they found a donor with the mutation, they used that bone marrow to treat the patient. Not only did the leukemia disappear, but so did the HIV.

"As of today, more than 20 months after the successful transplant, no HIV can be detected in the patient," the clinic said in a statement.

"We performed all tests, not only with blood but also with other reservoirs," Schneider told a news conference.

The researchers stressed that this would never become a standard treatment for HIV. Bone marrow stem cell transplants are rigorous and dangerous and require the patient to first have his or her own bone marrow completely destroyed.

Patients risk death from even the most minor infections because they have no immune system until the stem cells can grow and replace their own.

HIV has no cure and is always fatal. Cocktails of drugs can keep the virus suppressed, sometimes to undetectable levels. But research shows the virus never disappears - it lurks in so-called reservoirs throughout the body.

The CCR5 mutation is found in about 3 percent of Europeans, the researchers said. They said the study suggests that gene therapy, a highly experimental technology, might someday be used to help treat patients with HIV.

Questions:

1. How many months after the successful transplant have doctors still detected no HIV in the patient?

2. In what percentage of Europeans is the CCR5 mutation, which helps the body resist AIDS, found?

3. Why do researchers stress this will never become the standard treatment for HIV?

Answers:

1. 20 months.

2. About 3%.

3. Bone marrow stem cell transplants are rigorous and dangerous and require the patient to first have his or her own bone marrow completely destroyed, meaning they risk death from even the most minor infections.