A genetic mutation common in U.S. whites slows the progression of AIDS in people infected with the AIDS-causing virus and, in some cases, protects individuals against the disease, according to a new study.

The new work shows that about 1 in 7 U.S. whites and about 1 in 59 U.S. blacks inherit the protective, mutant gene from one parent, along with a normal copy of the gene from the other parent. This harmless genetic condition naturally slows the progress of AIDS, giving an average of three extra years of life to those infected with HIV, according to the study in today's issue of Science.

The work is the latest in a rapid-fire series of discoveries since June that has shifted AIDS researchers' attention away from the virus itself and onto a protein studding the surface of some human cells. The protein, known as CKR5 or CCR5, is part of a portal system that allows the AIDS virus to enter cells of the immune system. People with mutant CKR5 genes cannot make the crucial portal protein, and hence are protected to varying extents from viral invasion.