Deoxyadenosine triphosphate (dATP) is a purine nucleoside triphosphate used in cells for DNA synthesis. A nucleoside triphosphate is a molecule type that contains a nucleoside with three phosphates bound to it. dATP contains the sugar deoxyribose, a precursor to DNA synthesis whereby the two existing phosphate groups are cleaved with the remaining deoxyadenosine monophosphate being incorporated into DNA during replication. Due to its enzymatic incorporation into DNA, photoreactive dATP analogs such as N6-[4-azidobenzoyl(2-aminoethyl)]-2-deoxyadenosine-5-triphosphate (AB-dATP) and N6-[4-[3-(trifluoromethyl)-diazirin-3-yl]benzoyl-(2-aminoethyl)]-2-deoxyadenosine-5-triphosphate (DB-dATP) have been used for DNA photoaffinity labeling. When present in sufficiently high levels, dATP can act as an immunotoxin and a metabotoxin. An immunotoxin disrupts, limits the function, or destroys immune cells. A metabotoxin is an endogenous metabolite that causes adverse health effects at chronically high levels. Chronically high levels of deoxyadenosine triphosphate are associated with adenosine deaminase (ADA) deficiency, an inborn error of metabolism. ADA deficiency damages the immune system and causes severe combined immunodeficiency (SCID). People with SCID lack virtually all immune protection from bacteria, viruses, and fungi. They are prone to repeated and persistent infections that can be very serious or life-threatening. These infections are often caused by "opportunistic" organisms that ordinarily do not cause illness in people with a normal immune system. The main symptoms of ADA deficiency are pneumonia, chronic diarrhea, and widespread skin rashes. The mechanism by which dATP functions as an immunotoxin is as follows: a buildup of dATP in cells 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 unable to divide and propagate to respond to immune challenges.