Genetic engineering can make immune cells resistant to infection with human or monkey (simian) immunodeficiency virus (HIV and SIV, respectively). Recently, treatment with HIV-resistant immune cells has given encouraging results in patients. Stem cell researchers have now found ways of making large numbers of virus-resistant immune cells from monkeys, allowing future studies on safety and efficacy of immune therapy in a pre-clinical animal model. This encouraging research will help scientists consider alternative approaches to treating HIV, the virus causing AIDS.