It was the simplest idea but one with enormous potential. If a gene is defective in the human body, just replace it with one that works properly. Gene therapy would mean that genetic disorders would become a thing of the past. Cancer would be cured, as would cystic fibrosis and hundreds of other genetic illnesses. Scientists were justifiably excited about the idea but, this enthusiasm that would end up costing one young man his life.Jesse Gelsinger was born with a liver disorder, a rare condition called ornithine transcarbamylase (OTC) deficiency that stops the liver metabolising ammonia. People with the disease can suffer from brain damage or coma. At its most extreme the illness is fatal.Jesse was lucky, able to lead a fairly normal life although he had a daily cocktail of drugs to control his condition. Jesse wanted to help others. When he was offered a chance to take part in a medical trial to test the safety of using gene therapy for OTC deficiency, he was keen to participate. He knew this was not a cure for his condition but that, by volunteering he might be able to help others in the future.Although the concept of gene therapy is simple, the practice of administering the treatment is much more difficult. In order to replace defective genes, doctors must get working ones into the body and to the place where they are needed.