Gene therapy is an experimental technique for treating or preventing diseases by inserting a gene into a patient's cells. Genes are the basic unit of heredity. They carry the chemical instructions that determine the form and function of each cell. Through gene therapy, doctors can provide a new set of instructions for treated cells.

Every human cell has an estimated 50,000 to 100,000 genes. Genetic diseases result when a gene is defective or missing, causing affected cells to malfunction. Gene therapy makes it possible to correct such defects in the function of cells. Gene therapy is in the early stages of development but offers hope of treating or preventing genetic diseases that today are incurable. There are thousands of known genetic diseases. They include muscular dystrophy, cystic fibrosis, hemophilia, and some forms of cancer and cardiovascular disease.

Doctors first used gene therapy as a treatment in 1990. The case involved a 4-year-old girl whose deficiency of the enzyme adenosine deaminase (ADA) caused her immune system to become defective. Scientists at the National Institutes of Health in Bethesda, Maryland, inserted a normal human ADA gene into immune cells taken from the girl's body. They returned the treated cells to her body through a transfusion. The inserted gene instructed the cells to make normal amounts of the missing enzyme, and her defective immune system began to recover. Since 1990, scientists have used gene therapy to treat other genetic conditions, including cystic fibrosis, brain tumors, and a deadly skin cancer called melanoma.

Excerpt from the "Gene therapy" article, The World Book Encyclopedia © 1999