Doctors removed his bone marrow - the part of the body that makes blood. They then genetically altered it in a lab to compensate for the defect in his DNA that caused the disease. The altered bone marrow has been making healthy red blood cells for 15 months. He is no longer on any medication.
The doctors are reluctant to call it a cure, but the sickle cell disease is in full remission.
Sickle cell disease causes normally round red blood cells, which carry oxygen around the body, to become shaped like a sickle.
These deformed cells can lock together to block the flow of blood around the body. This can cause intense pain, organ damage and can be fatal.
The teenager who received the treatment had so much internal damage he needed to have his spleen removed and his hips replaced.