programmed cell death). The closely related thio-deoxyguanosine triphosphate (TdGTP) is built into DNA. [ 52 ] [ 55 ] Thioinosinic acid impedes later steps of DNA synthesis via enzymes such as adenylosuccinate synthase and IMP dehydrogenase . Moreover, azathioprine blocks the downstream effects of CD28 costimulation, a process required for T cell activation.