THE cells that give rise to the human immune system have been genetically altered to make them resistant to the most common strain of HIV.

"The results are very promising and the researchers have every right to be excited," says Dana Carroll, a biochemist at the University of Utah. The catch is that the therapy might help only some HIV-positive people even if it works as intended.

The idea is to block HIV's main route into cells, which is via a surface protein called CCR5. A few people produce a mutated, shortened form of CCR5 that never reaches the surface of cells, which makes them resistant to HIV infection without any ill effects.

Now a team at Sangamo BioSciences in Richmond, California, has shown that it is possible to induce a similar mutation in human cells. The approach relies on zinc fingers, proteins that bind to sequences of three DNA ...