Each gene is specific for an enzyme. Therefore for synthesis of every enzyme one gene is required which must be specific for that enzyme. In the absence or defect of a particular gene certain enzyme required to change a metabolic product for the next step to start with not be synthesised. As a result the chain of metabolic reaction which normally ends with production of a substance essential for functioning of the life system stops without completing the chain of reaction. This give rise genetic disease which normally cannot be cured. Gene therapy aims to remove such defect at the embryonic state by injecting correct gene. Permanent cure is still not possible in adult. Bone marrow transplantation, enzyme replacement therapy, infusion of genetically engineered lymphocytes of blood do not bring permanent cure unless treated at embryonic stage.

In the above scheme the substrate ‘C’ has not changed because the enzyme needed to change it to the next product is not available due to the defect of the gene responsible for its production. Therefore the substrate “C’ will accumulate in the body causing suffering which cannot be cured by any drug. Only gene therapy at the embryonic shape can cure it completely.