In 1972, Dr Hilaire Meuwissen of Albany, New York, sent some blood samples to Eloise Giblett of the Puget Sound Blood Center, Seattle, Washington, for analysis of genetic markers. One of the blood samples was from a patient with severe combined immunodeficiency disease and the other samples were from the patient’s parents Dr. Meuwissen was prepared to perform a bone marrow transplant in the patient and in preparation for the eventual necessity of documenting cell chimerism, was attempting to obtain as many genetic markers as possible to prove establishment of a bone marrow graft. Much to Dr. Giblett’s surprise, one of the enzymes, adenosine deaminase (ADA) (EC 3.5.4.4) was absent while the parents had approximately one-half normal activity. Several weeks later, Dr. Flossie Cohen of Detroit sent a blood specimen to Dr. Giblett from an additional patient with severe combined immunodeficiency disease. This patient also lacked ADA activity, while the parents had levels lower than normal. In 1973, a symposium on ADA deficiency and severe combined immunodeficiency disease was held in Albany, New York. After a careful review of laboratory and biochemical data, it was determined that 12 of 22 patients with SCID who had been tested for ADA activity in North American and Western Europe were ADA-deficient. Also in 1973, Jenkins reported finding a !Kung aborigine boy with absent red cell ADA activity but with normal function. Subsequently it was determined that the patient’s lymphocytes and fibroblasts had 10–30% of normal ADA activity. Recent reviews summarize these and additional cases [24].