To make up for the genetic error, scientists may try to replace a missing or ineffective protein with a drug or with the normal protein. Such experiments are usually carried out first in cultured cells in the laboratory, then in animals, and finally in humans.

Another option is gene therapy. Some scientists "infect" the defective cells with a virus into which they have inserted normal genes. Others use non-viral methods or even inject DNA directly into cells. Experiments that work in cultured cells are tried in animals and then in humans. For example, a patient's bone-marrow cells may be removed, treated with normal genes, and returned to the patient.

RESULT: Treatments are being developed for some genetic diseases. People will always carry genetic defects, but in the future, prevention and treatment will vastly reduce suffering from genetic diseases.

— Maya Pines

Illustrations: Stansbury Ronsaville Wood, Inc.

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