For the first time, researchers show that it is safe and effective to genetically modify cells to increase resistance to HIV infection in humans. They used a zinc-finger nuclease to target and inactivate the HIV coreceptor CC-chemokine receptor 5 (CCR5) in CD4+ T cells, which were then infused into 12 patients with chronic aviraemic HIV infection who were receiving antiretroviral treatment. The modified T cells readily engrafted and persisted (with a mean half-life of 48 weeks) in all 12 individuals. One individual suffered an adverse reaction to the transplant. Analysis of viraemia following the interruption of antiretroviral treatment in six participants revealed that the modified T cells had a survival advantage over unmodified T cells. Strikingly, the virus remained undetectable in one patient for the period of treatment interruption. This patient was later found to be heterozygous for the CCR5 deletion, which suggests that this therapy could be most effective in individuals with only one functional copy of CCR5.