Gene therapy is the insertion of a gene into an individual's cells to prevent or treat serious diseases. The inserted gene may be a healthy copy of a mutated gene, or it may be a different gene that counteracts the effects of a faulty gene. Genes can be delivered in vivo, directly into a person, or ex vivo, into cells removed from the body, which are then transplanted back.The new gene is inserted using a vector, typically a modified virus that can enter a cell and deliver the DNA to the nucleus. Some vectors integrate the DNA into the genome, while others remain outside the chromosome. The vectors contain a promoter region so that the gene can be transcribed. The introduced gene is then translated into protein providing a treatment for the disease.