T-cell acute lymphoblastic leukemia (T-ALL) is a relatively uncommon disease, constituting only approximately 15% of newly diagnosed acute lymphoblastic leukemias (ALL) in the United States, or roughly 300 cases per year. Outside of the United States, in countries such as Egypt and India, T-ALL may represent as much as 50% of all ALL's but still remains an overall rare disease. The clinical importance of T-ALL lies in its poor responsiveness to therapy that has proved highly effective with standard B-cell precursor ALL (BCP-ALL). The scientific importance of human T-ALL has resided in its role as a cancer prototype, permitting the identification of novel genes centrally involved in both neoplastic change and normal cellular differentiation. One of these genes, SCL/tal, has received significant attention due to its intimate involvement in T-ALL, as well as in normal hematopoiesis. Although a tremendous amount has been recently discovered about SCL/tal, its exact roles in leukemogenesis and normal hematopoiesis remain obscure.