Gilteritinib induces 2 distinct morphologic patterns of response in R/R FLT3^mut+AML: responses with and without differentiation. (A) Representative core biopsy marrow specimens from prior to initiation of gilteritinib and at the time of best marrow response show the effect of gilteritinib on total marrow cellularity in a subject with a differentiation response (top row) and in a subject with response without differentiation (second row). Differentiation responses are generally characterized by persistently hypercellular marrows, while responses without differentiation are associated with a reduction in total marrow cellularity. The core biopsy specimens are hematoxylin and eosin–stained at 400× objective magnification. (B) Bone marrow aspirate morphology at baseline and after treatment with gilteritinib in a subject with a differentiation response shows marked granulocytic hyperplasia with a left shift. The aspirate smears are Wright-Giemsa–stained at 500× objective magnification (oil immersion).