The alterations of transcription factor genes by chromosomal translocations play an important role in leukemogenesis and lymphomagenesis. The alterations are classified into two groups. One is the chimeric gene formation, and the other is the aberrant expression without structural changes. The former type is associated with the chromosomal translocations found in acute myeloid leukemia, such as the AML1/MTG8 in t(8;21) and PML/RAR alpha in t(15;17). The latter is the main mechanism in the gene activations observed in acute lymphoblastic leukemia and lymphoma. Many transcription factor genes are activated by the recombination with the immunoglobulin genes in B cell malignancies or T cell receptor genes in T cell malignancies. We isolated the AML1/EVI-1 fusion gene generated by the t(3;21) translocation, which is usually found in blastic crisis of chronic myelocytic leukemia. The chimeric transcription factor encoded by the fusion gene has dual functions, namely differentiation block and stimulation of proliferation. These findings provide new insight into the molecular mechanism in leukemogenesis by the chimeric transcription factors.