Adenosine deaminase (ADA) deficiency: A genetic (inherited) condition that results in a immune deficiency disorder called severe combined immunodeficiency disease. Adenosine deaminase is an enzyme that plays a key role in salvaging purine molecules.

ADA deficiency is of special interest in the history of genetics. The first successful instance of gene therapy in humans was carried out in 1990 by Drs. W. French Anderson, R. Michael Blaese and Kenneth W. Culver who infused genetically engineered blood cells to repair ADA deficiency.