A gene is usually __________.
1 Gene therapy uses sections of DNA (usually genes) to treat or prevent disease. 2  The DNA is carefully selected to correct the effect of a mutated gene that is causing disease.3  The technique was first developed in 1972 but has, so far, had limited success in treating human diseases. A section of DNA/gene containing instructions for making a useful protein is packaged within a vector, usually a virus, bacterium or plasmid. 2  The vector acts as a vehicle to carry the new DNA into the cells of a patient with a genetic disease.