SIRT4 and neoplasm: Thus, both SIRT1 and SIRT2 are activated in cancers, acting by diminishing tumour suppressors and stabilising oncogenes, or by activating glucose-6-phosphate dehydrogenase to increase NADPH production for tumour cell proliferation [105], but SIRT3 and SIRT4 can also protect cells against death by maintaining mitochondrial NAD+ levels following stress ([111]; see also the detailed discussion in [112]).