The first clinical gene therapy was tested in 1990 in the case of:AAdenosine deaminase deficiencyBCystic fibrosisCEmphysemaDPhenylketonuriaAnswer: A. Adenosine deaminase deficiency Read Explanation: ADA deficiency impairs the development and function of immune cells called lymphocytes.Lymphocytes are white blood cells that help the body fight infections. As a result, people with ADA deficiency often develop pneumonia, chronic diarrhea, and widespread skin rashes. Read more in App