A teenager suffering from

Doctors at Necker Children’s Hospital in Paris altered the genetic instructions in the 13-year-old's bone marrow.

This was said to have been done through the injection of a virus (which corrected the defect he had) into his bone marrow.

The altered bone marrow was then put back into the patient.

According to the results published in the New England Journal of Medicine, 15 months since the treatment, the patient no longer requires medication, and his blood cells show no further sign of the disease.

"So far the patient has no sign of the disease, no pain, no hospitalisation. He no longer requires a transfusion so we are quite pleased with that,"

Philippe Leboulch, a professor of medicine at the University of Paris, told the BBC.

"But of course we need to perform the same therapy in many patients to feel confident that it is robust enough to propose it as a mainstream therapy," he added.

BBC reports that Leboulch was also hesitant to use the word "cure" because the French teenager is only the first patient to scale through the clinical trials.