Adenosine Deaminase (ADA) is an enzyme, needed by body to develop lymphocytes of the immune system. Children who are born in mutations with mutations in ADA gene have severe combined immunodeficiency (SCID). Children with ADA-deficient SCID generally die in the first year of life from severe infections because they do not have immune system that can fight against infections. ADA deficient individuals can be cured by bone marrow transplant, but the best results are obtained when there is fully matched family donor is available. In the absence of a matched related donor, transplants from unrelated or mismatched donors have a much worse outcome. There is a form of enzyme therapy (PEG-ADA) for ADA-deficient SCID, in which children receive injections of purified ADA enzyme 1-2 times each week. These injections can allow the immune system to recover to a level that protects children from infections.