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The global HIV epidemic continues to spread: 40 million people are infected worldwide. While drugs are essential in the battle against HIV, a vaccine would be a major advance. A vaccine, for example, can be preventive and does not require frequent dosing. HIV's ability to evolve rapidly is a major hurdle in developing a vaccine. HIV replication uses a reverse transcriptase enzyme that converts viral RNA into DNA. The enzyme is poor at reading and correcting mistakes. With successive replication cycles, alterations in viral genes accumulate, resulting in the evolution of new viral traits. HIV shows more variability in a single person than the total viral variability seen across a global influenza epidemic. This rapid HIV evolution makes it difficult to pick a stable protein sequence to target for vaccine development. Currently, the focus is on keeping HIV in check rather than developing a completely preventive vaccine. Individuals whose native immunity has kept HIV under control for more than 25 years may provide clues for creating a vaccine.