ADA deficiency: Lack of normal adenosine deaminase (ADA) activity, a genetic (inherited) condition causing one form of severe combined immunodeficiency ((SCID) disease. It is said to be "combined" in that there is dysfunction of both B and T lymphocytes with impaired cellular immunity and decreased production of immunoglobulins. Adenosine deaminase is an enzyme that plays a key role in salvaging purine molecules.

The ADA gene is located on chromosome 20 in the region of bands q12-q13.11. The complete sequence and structure of the gene is known. A variety of mutant alleles (altered forms of the gene) have been identified, including basepair substitutions and deletions.

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.