Excerpted from NOVA: Surviving AIDS, this video segment showcases the pioneering work of Drs. David Ho and Stephen O'Brien in the 1990s. By examining the "outliers"—in this case, people like Steve Crohn, whose cells repeatedly resisted HIV infection—Ho and his colleagues found a genetic mutation that prevents the HIV virus from entering the cell. This video segment includes animation of HIV entering a white blood cell through the CD4 and CCR5 receptors on the cell's surface. Some individuals have no CCR5 gene, which means that HIV cannot enter their cells. Scientists have since used this information in the development of AIDS treatments.