Neutrophils are derived from the leukemic blasts. (A) Graph of peripheral blood absolute neutrophil count from patient 4 during treatment with quizartinib. Neutrophils from day 60 were isolated to > 95% homogeneity as described in “Methods,” which was confirmed by cytospin (inset). (B) Genomic DNA samples isolated from pretreatment blasts, day 60 neutrophils, and neutrophils from a healthy donor were analyzed by PCR for the FLT3/ITD mutation, as described in “Methods.” Shown is an ethidium bromide–stained agarose gel. WT indicates wild-type. (C) Whole-cell lysates were prepared from Molm14 cells, pretreatment blasts from patient 4, day 60 neutrophils from patient 4, and neutrophils from a healthy donor. Lysates were analyzed for total FLT3, phosphorylated FLT3, lactoferrin, and MMP9, as described in “Methods.” (D) NBT reduction assay of peripheral blood collected from a healthy donor (“Control”) compared with a quizartinib-treated patient during the neutrophil surge. Cells “with stimulation” were exposed to bacterial extract to induce respiratory burst activity.