with no more risk of catching infections than they had.

Blood cells can be genetically altered and reintroduced to the body by a simple injection into a blood vessel. But what if you want to alter genes in the cells of an organ, such as the liver? One approach is to remove a piece of the liver, divide it into individual cells, insert the appropriate genes into each cell, and transplant the engineered cells back into the patient.

A second approach, according to William French Anderson and others, is to develop smart vectors—ones that can find their own way to diseased tissue inside the body. Rather than inserting genes into cells in petri dishes, the new generation of vectors will be injected directly into patients to carry genes to their targets like guided missiles. This could be achieved by attaching molecules to the vector that recognize specific proteins found on the surface of cells in the target organ.

The type of gene therapy I've described adds normal genes to a patient to produce something the patient lacks due to genetic defects. Another type of gene therapy works in a different way, by obstructing genes that cause disease. In this strategy, called antisense therapy, scientists add a gene that mirrors the target gene—say, one that causes arthritis. The engineered gene produces RNA that complements the RNA of the troublesome gene, binding onto it and blocking its action. So, for example, if the disease-causing gene produces an unwanted protein, antisense therapy will prevent the protein from being formed. If the gene suppresses the formation of a wanted protein, the therapy will allow for normal protein production.

The first stage of gene therapy—identifying genes associated with disease—is fairly well established, thanks to the Human Genome Project. News reports frequently