Here, we show that ex vivo drug sensitivity profiling predicts clin. response of a relapsed pediatric mixed-phenotype acute leukemia (MPAL).MPAL refers to a heterogeneous group of leukemia with either discrete admixed populations of myeloid and lymphoid blasts.They account for 2-5of all acute leukemia, and typically have a worse prognosis than their single-lineage counterpart acute lymphoblastic (ALL) or acute myeloid leukemia (AML).Given the lack of consensus clin. guidelines, treatment is usually based on either standard ALL or AML therapy, depending on principal lineage marker expression and often includes allogeneic hematopoietic stem cell transplantation (HSCT).A 12-yr-old male with a white blood cell (WBC) count of 21.2 × 109/L and 82of blasts was diagnosed with MPAL, classified as bilineage early T-cell precursor (T-ALL, 40) and myeloid megakaryoblastic (AML-M7, 60).Correspondingly, cytogenetics showed two major clones (Figure S1).After receiving three lines of intensive chemotherapy, successful salvage therapy reduced minimal residual disease (MRD) down to 10-4, allowing HSCT (Supporting Information).The child relapsed 2.8 years after initial diagnosis, blood count showed 85blasts (WBC: 17 × 109/L, Hb: 132 g/L, platelets: 178 × 109/L). Bone marrow (BM) anal. established disease relapse with 79myeloperoxidase-pos. myeloblast infiltration classified as AML-M2.Importantly, the T-lineage ALL clone was not detected.The child was refractory to the first-line relapse therapy of fludarabine/cytarabine/idarubicine/gemtuzumab ozogamicin (Fla+Ida+GO, Figure S1).Ex vivo drug testing has the potential to become a useful complementary clin. tool in the field of precision hemato-oncol. and treatment stratification.Significant efforts should thus be made to demonstrate the clin. benefit of ex vivo drug profiling in larger prospective studies.