It is important to note that if an acute leukemia exhibits a T cell immunophenotype (including expression of the T cell lineage-defining antigen CD3) and exhibits high-level expression of the myeloid lineage-defining marker myeloperoxidase (MPO), monocytic lineage-defining markers, or sufficient B lineage-defining markers, then a diagnosis of mixed-phenotype acute leukemia (MPAL), discussed elsewhere in this publication, is favored [8]. This evidence concerns the gene MPO and acute leukemia.