In accordance with previous reports of selective efficacy of FLT3 inhibitors in MLL-rearranged ALL with high levels of FLT3 [89], we provide compelling in vivo data validating bulk FLT3 expression as a predictive biomarker for molecular response to gilteritinib in ETP-ALL irrespective of FLT3 mutation status. The gene discussed is FLT3; the disease is acute lymphoblastic leukemia.