TdT positive and subsequently occurred TdT negative blast crises were observed in a 49-year-old patient with Ph1-positive CML. In the first crisis, blast cells of the patient showed lymphoid morphology (L1) and they had high TdT activity and surface markers of non-T cell, such as negative surface immunoglobulin, positive C3-receptor, Fc-receptor and Ia-like antigen. In the second crisis, however, the blast cells were considered myeloblasts (M1) based on morphologic impression and showed no TdT activity. Both crises responded well to VP therapy. This case suggests that loss of TdT activity in blast cells does not always imply the emergence of cells resistant to VP therapy. The relationships between TdT, VP therapy and karyotype and the origin of target cell in the blast crisis were discussed.
