Adenosine deaminase (ADA) deficiency is an inherited disorder characterized by repeated, persistent, and potentially life-threatening infections. Symptoms are due to a defect in the production of an enzyme called adenosine deaminase. Without normal levels of this enzyme, toxic chemicals build up within the immune system leading to severe immunodeficiency, and the body's ability to fight infectious disease is absent or severely compromised. The severe form of ADA deficiency is also known as ADA-deficient severe combined immunodeficiency (SCID). Children with SCID lack virtually all immune protection from bacteria, viruses, and fungi. Affected children can have growth and developmental delay. Most individuals with ADA deficiency are diagnosed with SCID in the first 6 months of life. Without treatment, these babies usually do not survive past age 2 because of life-threatening infection. In about 10 percent to 15 percent of cases, onset of immune deficiency is between 6 and 24 months of age (delayed onset) or even until adulthood (late onset). Once diagnosed, most patients affected by SCID can be cured by bone marrow transplantation from a donor, though there are risks with the procedure. Other methods, including enzyme replacement for the defected adenosine deaminase, and gene therapy, are in clinical trials. Adenosine deaminase deficiency is caused by pathogenic variants in ADA gene.