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Cancer cells are frequently deficient in a DNA repair gene. (Also see DNA repair deficiency in cancer.) This DNA repair defect either may be due to mutation or, often, epigenetic silencing (see epigenetic silencing of DNA repair). If this DNA repair defect is in one of seven DNA repair pathways (see DNA repair pathways), and a compensating DNA repair pathway is inhibited, then the tumor cells may be killed by synthetic lethality. Non-tumorous cells, with the initial pathway intact, can survive.