Kathmandu: A single change in a human gene may hold the key to preventing people living with HIV from progressing to full-blown AIDS, researchers said on Monday. They found a crucial difference between a gene in humans and one in rhesus monkeys that block infection of the virus in the animals a finding that offers new insights into the origins of AIDS and gene therapy. Had the gene been the same in humans, scientists at the National Institute of Medical Research in London believe, there may not have been the AIDS epidemic that now affects 40 million people worldwide.