DAC-induced origin of DAC resistance. (A) Induction of the phosphorylation of the histone H2AX by DAC treatment. HL60D cells were treated with DAC (0.2 and 2 μM) for 3 days, and H2AX phosphorylation was measured by Western blot analysis. β-Actin was used as a control. (B) DAC treatment increased HRR rates. Cells with stable PLNCX-GZ transfection were treated with DAC for 4 days and maintained in drug-free medium for 1 week. Colonies resistant to Zeocin were selected and maintained in methylcellulose medium for 2 weeks. HRR rates were calculated as described in “Measurement of homologous recombination repair.” (C) Verification of intrachromosomal recombination in zeocin-resistant colonies by PCR with recombination-specific primers. PCR-amplified DNA from G418-resistant colonies produced a 2.2-kb fragment diagnostic for the nonrecombined tandem repeats. PCR-amplified DNA from G418- and zeocin-resistant colonies generated a diagnostic 1.1-kb fragment.