FAN1 and neoplasm: To address the possibility that nick-dependent FAN1 nuclease cleavage of extrahelical extrusions might lead to removal of such structures, we evaluated the fate of 3′(CAG)2 or 5′(CAG)2 DNAs upon incubation with nuclear extracts of HCT116 tumor cells (MLH-/- and MSH3-/-) that are deficient in MutLα and MutSβ (46).