Adenosine deaminase 2 (ADA2) was first described as the residual source of adenosine deaminase activity in the spleen of a patient with severe combined immunodeficiency (SCID) due to adenosine deaminase deficiency (ADA; also known as ADA1) [3]. This evidence concerns the gene ADA2 and hyperinsulinemic hypoglycemia, familial, 4.