ADA deficiency is a genetic disorder characterized by insufficient activity of adenosine deaminase (ADA), leading to a type of severe combined immunodeficiency (SCID). This condition is termed "combined" because it involves dysfunction in both B and T lymphocytes, resulting in compromised cellular immunity and reduced immunoglobulin production. Adenosine deaminase is crucial for recycling certain molecules.The ADA gene is situated on chromosome 20, specifically between bands q12 and q13.11, and its entire sequence and structure have been mapped. Numerous mutant alleles, which are altered versions of the gene, have been discovered, including those resulting from base pair substitutions and deletions.ADA deficiency has played a significant role in the history of gene therapy. The first successful human gene therapy was conducted in 1990 by Drs. W. French Anderson, R. Michael Blaise, and Kenneth W. Culver, who introduced genetically modified blood cells to correct ADA deficiency. Clinical trials for treating this condition are ongoing, and bone marrow transplantation during infancy has shown to be effective.