A missing chunk of DNA, 32 base pairs long and smack in the middle of the CCR5 gene, might be the most studied mutation in human history. The spontaneous deletion, which arose thousands of years ago, has a striking relationship with one of the worst human diseases: HIV/AIDS. People who inherit this mutation from both of their parents are naturally immune. The only two people to have ever been cured both received bone marrow transplants from people who carry the Δ32 mutation.