TCL1B and leukemia: The protooncogene TCL1 (T-cell leukemia 1) family protein was first identified in the translocation of T-cell prolymphocytic leukemia (T-PLL), a rare form of human adulthood leukemia.8, 9 The physiological expression of TCL1 is tightly limited to early developmental cells as well as various developmental stages of immune cells.