The treatment of genetic disease has developed over the past thirty years, and includes: the classical approach, which aims to prevent organ damage from excessive amounts of metabolites generated by the metabolic defect or which adds substances that are not synthesised; enzyme replacement therapy, which attempts to introduce a specific enzyme with a normal configuration into organs where the enzyme is missing or malfunctioning because of structural abnormalities; the transplantation of cells, especially bone marrow cells, and whole organs; and somatic gene therapy. The clinical outcomes of attempts to treat genetic disease have only been partially successful. More developmental work is required before the therapies can be said to be effective, especially in the prevention of brain damage.