Adenosine deaminase (ADA; EC 3.5.4.4) is a ubiquitous enzyme that catabolizes adenosine and deoxyadenosine to inosine and deoxyinosine. During an ischemic brain event, the extracellular adenosine concentration increases over 10-fold and adenosine is deaminated to inosine by ADA. Congenital ADA deficiency results in severe combined immunodeficiency (SCID), caused by an increase in deoxy-adenosine levels in the serum and tissues. This increases deoxy-ATP levels in the T-cells and causes T-cell apoptosis, and primarily affects lymphocyte development, viability, and function when diagnosed in infancy.