Expression of terminal deoxynucleotidyl transferase in malignant myeloblasts.

Blast cells from untreated cases of acute leukemia were examined for expression of terminal transferase enzyme (TdT) by immunofluorescence and for myeloid lineage commitment as demonstrated by the presence of myeloperoxidase enzyme at the ultrastructural level. In three cases, an overlapping expression of the "lymphoid-specific" marker TdT was found on peroxidase-positive myeloblasts. These results indicate either the clonal expansion of a rare TdT-positive myeloid precursor or inappropriate expression of TdT by malignant myeloblasts, and further illustrate that TdT expression must be interpreted with caution when distinguishing between lymphoid and myeloid leukemias.