The document defines asynchronous antigen expression as antigens expressed during normal maturation that are found simultaneously on malignant cells, which can lead to misinterpretation and incorrect treatment if not properly understood. Aberrant antigen expression is defined as antigens unexpected for the lineage in question but that do not change the lineage assignment. The diagnosis of acute leukemia of ambiguous lineage is based on flow cytometric immunophenotyping showing no clear evidence of differentiation along a single lineage.