The gene, called RANTES, can significantly increase an individual's chances of being infected with HIV.
But it has also been found to slow down the progression from HIV to Aids.
The discovery has lead to hopes that scientists may be close to discovering why some people are immune to disease and why others are more susceptible.
This in turn could assist scientists who are working on HIV treatments and possible cures.
Researchers in the US suggest that if they could develop a drug that mimicked the gene they could stop HIV from developing into Aids for many people.
They discovered that tiny differences in the RANTES gene, which is part of the immune system, can double a person's susceptibility to HIV.
However, the same variation also means that an individual infected with HIV will take about 40% longer to develop Aids.
Dr Anthony Fauci, director of the US National Institute of Allergy and Infectious Diseases (NIAID), described the finding as a "first".
"This study offers the first genetic evidence that RANTES affects the risk of HIV transmission.
"It also adds to the evidence that RANTES can slow the progression to Aids in HIV-positive individuals, lending support to the search for a drug that mimics this gene's action."
The research has prompted some companies to begin work on developing a RANTES-based treatment that could be used to slow the progression rate of HIV in some patients.
Scientists have already identified the gene which makes an individual practically resistant to HIV.
That gene, called CCR5, prevents the T-cell receptors that HIV uses from being made in the first place.
However, Dr David McDermott, who led this latest research, said that the discovery does not mean that people with this gene are immune.
"If you stand in the way of a speeding truck, the risk of death is very high," he said.
The study is published in the Journal of Aids.