Rearrangement of the tal-1 gene is the most frequent clonal marker in childhood T cell acute leukemia. Previously, tal-1 mRNA expression has been observed only in cells of the erythroid, mast cell, and megakaryocytic lineages and in blastic lymphoid cells of normal bone marrow, not in normal lymphocytes or monocytes of the peripheral blood (PB). In this study we addressed the question of tal-1 expression during normal hematopoietic development by performing reverse transcription-polymerase chain reaction (RT-PCR) on RNA from PB cells of 12 healthy donors. Ten of 10 unsorted samples were RT-PCR positive for tal-1 expression. Sorted T cells and monocytes from three donors showed tal-1 RT-PCR products. This is the first direct experimental evidence of tal-1 transcripts in these two normal PB cell types.
