III.

Types of Gene Therapy

There are two distinctly different types of gene therapy: somatic-cell therapy and germ-line therapy. In somatic-cell therapy, gene surgeons attempt to fix genetic malfunctions in somatic (body) cells, such as blood cells and skin cells. This type of gene therapy is the only one that has been performed on humans. Genetic alterations to somatic cells are restricted to the person being treated and cannot be passed on to his or her offspring.

In germ-line therapy, genetic alterations are made to germ cells, such as sperm and eggs, in order to treat inherited diseases. Germ-line therapy is highly controversial because such changes would alter the genetic endowment of the unborn and could be passed on to future generations. Germ-line experiments with laboratory animals have been performed since the late 1980s. In countries that regulate gene therapy in clinical trials, germ-line therapy has yet to be applied to humans. Nevertheless, U.S. researchers have submitted proposals to conduct human germ-line experiments, and many experts predict the first of these experiments will occur early in the 21st century.