Increase in H2AX phosphorylation in leukemia cells obtained from bone marrow. The bone marrow blasts were obtained longitudinally from 4 patients and the presence and magnitude of damaged DNA was measured by flow-cytometric analysis on day 0 (Pre-Study), EOI CY (D0 End CY), and day 3 clofarabine followed by CY (D3 End). Cellular debris was gated out and nonspecific staining controls were used to mark the lower limit of the positive γH2AX-staining region (SR). Control SR was subtracted from sample SR to determine the percentage of cells staining positive for γH2AX (expressed as % γH2AX). Pretreatment value was expressed as 0 and all other values were percentage change.