Scientists say they have uncovered an important clue to understanding the origins of the Aids epidemic.
Researchers compared human and monkey genes
They have pinpointed crucial differences in a gene found in rhesus monkeys that can prevent HIV infection, and its human counterpart, that cannot.
It appears that only a single change to the human gene is needed to enable it to block HIV infection.
The study, by the National Institute for Medical Research, is published in Current Biology.
The scientists say their work indicates that HIV would not have become established in the human population if mankind carried the same version of the gene found in rhesus monkeys.
Lead researcher Dr Jonathan Stoye said: "This discovery has significant implications for the development of effective gene therapy to combat Aids.
"In theory, it should be possible to take cells from an HIV-infected individual, make them resistant to HIV infection with the modified gene and reintroduce them into the patient. These cells could then block progression to Aids.
"Alternatively we could seek for drugs that activate the human gene against HIV."
Long term approach
Christopher Gadd, editor of HIV & AIDS Treatments Directory, told the BBC News website: "What is particularly exciting is that the researchers have identified that changing just one amino acid 'building block' in one protein can switch a cell from susceptible to insusceptible to long-term HIV infection.
"However, it is important to stress that any therapeutic benefits that may arise from this research are unlikely to be felt for many years.
"This type of gene therapy would involve removing white blood cells from patients, cloning them, and altering their genetic make-up before reintroducing them to the patient on an individual-by-individual basis.
"Although it is theoretically possible, this approach is unlikely to be practical or cost-effective with currently available technologies."
Jo Robinson, a senior treatment specialist at The HIV charity Terrence Higgins Trust agreed that gene therapy was a promising approach which might yield results in the longer term.
But she said: "We should stress that although this research is important we are still a long way from it having a practical application for people with HIV. There is still no cure for HIV and no vaccine."
According to the latest UNAIDS figures, at the end of 2004, 39.4 million people worldwide - 37.2 million adults and 2.2 million children younger than 15 years - are living with HIV/Aids.
HIV is a retrovirus that infects cells of the human immune system and destroys or impairs their function.
Infection with this virus can result in the progressive depletion of the immune system, which ultimately may lead to Aids.