ERT has also been successful in treating severe combined immunodeficiency caused by an adenosine deaminase deficiency (ADA-SCID).[7] This is a fatal childhood disease that requires early medical intervention.[7] When the enzyme adenosine deaminase is deficient in the body, the result is a toxic build-up of metabolites that impair lymphocyte development and function.[7] Many ADA deficient children with SCID have been treated with the polyethylene glycol-conjugated adenosine deaminase (PEG-ADA) enzyme. This is a form of ERT that has resulted in healthier, longer lives for patients with ADA-SCID.[7]