FdUMP[10] causes S-phase arrest regardless of p53 status. (A) EdU (5-ethynyl-2′-deoxyuridine) incorporation assays. MLL-ENL–driven murine AML cells were incubated with the indicated drug for 8 hours and subjected to an EdU incorporation assay. S-phase cells were gated as shown. Percentages shown are for cells in S phase. (B) EdU incorporation assay as in panel A. Cells were infected with either a p53-targeting shRNA or a control vector and exposed to the indicated drug for 8 hours.