The first clinical gene therapy was given for treating Diabetes mellitus, Chicken pox, Rheumatoid arthritis, or Adenosine deaminase deficiency?

Gene therapy is the process of introduction of DNA into living human beings in order to treat disease. It is used to replace a missing gene product or to correct mutant alleles. The first human gene therapy trial was conducted at the National Institutes of Health in Bethesda, Maryland, in 1990 on a four-year-old girl suffering from ADA deficiency (Adenosine deaminase deficiency). A functional ADA gene was introduced ex vivo into the bone marrow cells of the child through an engineered retrovirus containing a functional ADA gene. The treated cells were reintroduced into the patient’s marrow. So, the correct answer is 'Adenosine deaminase deficiency'