Differentiation responses to gilteritinib are associated with stable total marrow cellularity and FLT3 estimated mutant allele frequency despite significant reduction in marrow blast percentage. (A) Differentiation responses are characterized by a significant reduction in marrow blast percentage with a relatively stable total marrow cellularity and FLT3 mutation frequency at the time of best marrow response compared with baseline. (B) Responses without differentiation are associated with a reduction in marrow blast percentage with a concomitant significant reduction in marrow cellularity and FLT3 mutation frequency. Subjects 9, 13, and 18 had both FLT3-ITD and FLT3-D835 mutations. Only FLT3-ITD mutation frequencies are shown here. See supplemental Figure 3 for changes in FLT3-D835 frequencies in these subjects. Subject 12 had variably cellular (0% to 90%) total marrow cellularity at baseline; thus, mean baseline cellularity (45%) is shown on the graph. FLT3 mutation frequencies are not shown for subject 12 (specimens were suboptimal for analysis) or subject 13 (baseline FLT3 polymerase chain reaction was not performed, as the subject’s FLT3 mutation was confirmed by a next-generation sequencing panel).