The gene in question codes for a protein receptor called CCR5. The HIV virus binds to this receptor before entering cells. One gene variant, called CCR5-Delta32, has 32 DNA base pairs missing and produces a receptor that HIV cannot bind to, which prevents the virus from entering the cells. People with this variant have some resistance to HIV infection and also take longer to develop AIDS. Generally, only people in Europe and western Asia carry the variant, and it becomes less and less frequent as you move south. For example, more than 15 per cent of people in some areas...