Of the 11 patients, 9 were cured after receiving an infusion of healthy genes, tucked inside a retrovirus.

The idea is to remove all the dangerous genes in the virus and inject the healthy human genes.

One way to deliver healthy genes is by placing them inside a tamed virus that in turn can enter the patient's affected cells.

The problem is getting the healthy genes into the patient.

The donated tissue provides a healthy gene in place of the mutated one.

The move is yet another major setback for the fledgling field of gene therapy, which involves using viruses to introduce healthy genes into cells.

The idea behind gene therapy is disarmingly simple: to treat or cure disease by giving healthy genes to patients with defective ones.

A healthy gene is cloned to take over for the defective one that causes Canavan disease.

The researchers have developed a method of delivering healthy genes directly into the brain.