The genetic scripts written in DNA code constantly undergo
changes, or mutations. At times, these mistakes in a gene's
message can be harmful; often, they have no significant effect.
Occasionally, though, a mutation confers a survival advantage in
the face of an environmental change. Most of the non-carriers of
the mutation die, and those with the mutation are able to
reproduce. With that powerful evolutionary selection force, the
gene can become common in a population. Recently, scientists
were astonished to find that some individuals did not become
infected with HIV, even after repeated exposure to the deadly
virus. For some reason, they were immune. A long and difficult
scientific search, using blood samples from hundreds of HIV-
resistant patients, finally teased out the genetic explanation.
Resistant individuals had in their cells two copies of a
mutation that disrupted the entryway through which HIV viruses
entered white blood cells. People who inherited just one copy of
the change could become infected,