This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Structure and function of protein, Subject name: Biochemistry
The first gene therapy (somatic enzyme) was successfully done in 
Select and explain. 
A. SCID, B. Phenylketonuria, C. Thalassemia, D. Cystic fibrosis
A: SCID
Answer: A: SCID 
Explanation: Deficiency of adenosine deaminase, a condition that affects lymphocytes paicularly, was the first disease to be treated by gene therapy. Deficiency of adenosine deaminase accounts for 15% of all cases of severe combined immunodeficiency (SCID)In 1990, Dr. William French Anderson introduced a new copy the gene, carried on a retroviral vector, into a 4-year-old girl suffering from severe combined immunodeficiency (Bubble boy disease).Ref: Harper 30th edition pg: 699