ADA deficiency can be further classified as early-onset (i.e. infants and small children) ADA-SCID; or less severe delayed (i.e. usually diagnosed between 1 and 10 years), or late-onset (i.e. diagnosed between 2nd to 4th decades) combined immunodeficiency (ADA-CID) [3]. The gene discussed is ADA; the disease is combined immunodeficiency.