A team of researchers has reported a novel method for tracking CD4+ T cells in people infected with HIV. CD4+ T cells are critical for immune defense against an array of pathogens and are a primary target of HIV. In the study, researchers used a unique, replication-incompetent (defective) form of HIV identified in a patient in the early 1990s. The defective virus had integrated into the genome of a single CD4+ T cell. Like a barcode, this "provirus" marked the originally infected CD4+ T cell and its progeny, enabling researchers to track its lineage for 17 years. This new method allows scientists to distinguish dividing cells from dying ones, something that has not been possible with existing labeling techniques, but is essential for studying how immune cells survive HIV infection.