Monkeys do not become sick with AIDS. Now some scientists say they have discovered why. They have identified a key protein that makes monkeys resistant to the disease. The finding could be an invaluable tool for studying the deadly virus, possibly leading to better treatments and even a cure.

The protein is called trim-5-alpha and, once AIDS has been injected into rhesus monkeys, trim-5-alpha thwarts HIV infection by refusing to relay the virus' genetic instructions to its DNA, triggering the process that leads to AIDS.

Right now, much research is focused on the later stages of HIV infection, once the virus has commandeered the body's immune system. The research has led to combination therapies that keep the disease at bay.

But finding trim-5-alpha could give researchers a tool to attack the disease at an earlier stage, before it really gets started. Virologist Joseph Sodroski of the Dana-Farber Cancer Institute in Boston oversaw the research that pinpointed trim-5-alpha as the primary agent in rhesus monkeys that causes resistance to AIDS.

"It's been observed over the years that HIV can enter monkey cells, but once they enter those cells, it encounters a very specific and potent block," he said.

Dr. Sodroski says humans also have genes that make trim-5-alpha, but at much lower levels. "I think there's going to be many people thinking of creative ways to try to potentiate the human factor, and stimulate our own natural resistance to HIV," he said.

Cellular biologist Stephen Goff of Columbia University in New York says the discovery of trim-5-alpha, by shining light on the earliest stages of the AIDS process, could eventually lead to drugs that interrupt the transformation of human HIV into AIDs.

"The most exciting potential is that this will tell us how to attack the virus at a novel stage, really a stage that would be very attractive to target, where we just haven't had enough information on how to proceed," he said.

Professor Goff's comments on the discovery of trim-5-alpha is published in the journal Nature, where the research appears.