In acute lymphoblastic leukemia (ALL), the blasts are morphologically undifferentiated. By cytochemistry, they show no relevant reaction with myeloperoxidase (<3%) or non-specific esterase (NSE). The final assignment to the lymphatic lineage and the discrimination from the AML subtypes FAB M0 or M7 requires the application of immunophenotyping in all cases.

In case there are blasts with vacuolized or basophil cytoplasm, the possibility of mature B-ALL (=Burkitt lymphoma) should always be considered, and the respective genetic analyses should immediately be initiated.