Most cases of acute leukemia can be assigned to myeloid, B, or T lineage according to morphologic features and immunophenotype. However, 3% to 5% of all cases of acute leukemias in adults constitute a heterogeneous subset that cannot be definitively assigned to a certain cell lineage.1 These entities have been referred to as acute leukemias of ambiguous lineage (ALALs). Acute undifferentiated leukemia (AUL) has been categorized by WHO as a rare subtype of ALAL2 that expresses no known lineage-specific markers but instead may express only HLA-DR, CD34, and/or CD38, and may be positive for terminal deoxynucleotidyl transferase (TdT).3 The overall prognosis of AUL is poor, and because of its low incidence, knowledge about AUL is limited in terms of clinical and biological characteristics. No standard treatment approach exists for patients with AUL.
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