Human chromosome translocation t(12;21)(p12;q22) is the most frequent chromosome rearrangement in childhood B-lineage acute lymphoblastic leukemia (ALL), and produces the TEL/AML1 fusion protein. The chimeric protein, TEL/AML1 contains, the first 336 amino acids of TEL that is linked to residues 21-480 of AML1 and the fusion protein is generally known as a transcription repressor to the various target genes. Furthermore, TEL/AML1 has been shown to interfere with AML1-mediated transactivation on the CR1 gene. To understand the mechanism of the TEL/AML1-mediated repression, we used transient-transfection assay and immunofluorescence to monitor subcellular localization of TEL/AML1. Here, we show that TEL/AML1 is localized in the cytoplasm and the transcriptional activities of CR1 promoter are affected by the subcellular localization of TEL/AML1 fusion protein. (C) 2002 Elsevier Science (USA). All rights reserved.