Having extra copies of a gene that produces a blocking protein helps protect people from AIDS (search), a finding that may explain why some people are more susceptible to the disease than others, a new study reports.

Researchers wondering why people from the same ancestry varied in their ability to resist HIV and AIDS found differences in the number of copies of the gene that encodes CCL3L1 (search), a protein that blocks HIV (search), the virus that causes AIDS.

Researchers hope the finding, reported in Thursday's online issue of the journal Science, help them identify people who have a higher or lower susceptibility to the disease.

"Individual risk of acquiring HIV and experiencing rapid disease progression is not uniform within populations," said Dr. Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, which funded the study.

"This important study identifies genetic factors of particular groups that either mitigate or enhance one's susceptibility to infection and disease onset," Fauci said in a statement.

The study looked at more than 4,300 blood samples from people of different ancestral origins to determine the average number of CCL3L1 gene copies in each group.

The researchers reported that, for example, HIV-negative African-American adults had an average of four CCL3L1 copies, while HIV-negative European- and Hispanic-American adults averaged two and three copies each.

They said this does not mean that one group is more prone to AIDS than another, only that people with fewer copies than their population's average were more susceptible to HIV infection while those with more than average copies were less prone to infection.

Senior authors of the study were Dr. Sunil K. Ahuja of the University of Texas Health Science Center and the Veterans Administration Center for AIDS and HIV-1 Infection in San Antonio and Dr. Matthew J. Dolan of the Air Force's Wilford Hall Medical Center and Brooks City-Base in San Antonio.

In addition to NIAID the research was supported by National Institute of Mental Health, the Veterans Administration Center for AIDS and HIV-1 Infection, the Elizabeth Glaser Pediatric AIDS Foundation and the Burroughs Wellcome Fund.