Nearly 4,000 genetic diseases afflict human beings. Given enough time and effort, scientists can learn to prevent or treat a great many of them. This requires answering three questions—major landmarks on a trail of genetic discoveries:

• What faulty gene causes the disease?
• What protein does this gene normally produce?
• Can the defective protein or gene be fixed or replaced?

Two different strategies may be used. Researchers may find the protein first (if it can be detected chemically in tissues that are affected by the disease) and then locate the gene that codes for it. When this is impossible, they use reverse genetics: They find the faulty gene first (by zeroing in on the DNA that is inherited with the disease, or by locating a similar gene in a mouse) and then identify the protein that it makes.

The trail shown here illustrates reverse genetics. This strategy has led to spectacular progress toward preventing or treating cystic fibrosis, Duchenne muscular dystrophy, neurofibromatosis, and other inherited disorders.

— Maya Pines

Illustration: Stansbury Ronsaville Wood, Inc.

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