ADA<unk>  is due to a lack of the enzyme adenosine<unk> . This<unk>  results in an accumulation of deoxyadenosine, which, in turn, leads to: * a buildup of dATP in all cells, which inhibits ribonucleotide reductase and prevents DNA synthesis, so cells are unable to divide. Since developing T cells and B cells are some of the most mitotically active cells, they are highly susceptible to this condition. * an increase in S-adenosylhomocysteine since the enzyme adenosine<unk>  is important in the purine salvage pathway; both substances are toxic to immature lymphocytes, which thus fail to mature. Because T cells undergo proliferation and development in the thymus, affected individuals typically have a small, underdeveloped thymus. As a result, the immune system is severely compromised or completely lacking. Diagnosis== The diagnosis is based on clinical features, with a concomitant decreased blood adenosine<unk>  level supporting the diagnosis.
Adenosine deaminase deficiency
Adenosine deaminase 2 deficiency
Adenine phosphoribosyltransferase deficiency
Guanidinoacetate methyltransferase deficiency
Galactose-1-phosphate uridylyltransferase deficiency
Methylenetetrahydrofolate reductase deficiency
Mevalonate kinase deficiency
Adenosine monophosphate deaminase deficiency type 1