Adenosine deaminase (ADA) deaminates adenosine and deoxyadenosine into inosine or deoxyinosine. ADA is part of the purine metabolism. In humans ADA is involved in the development and maintenance of the immune system. ADA deficiency is the cause of severe combined immunodeficiency (SCID). SCID was the first disorder to be treated by gene therapy. There are 2 isoforms of ADA: ADA1 and ADA2. ADA contains Zn+2 ion in its active site.