The TCL1 gene was recently cloned as a candidate target within the 14q32.1 breakpoint cluster region observed in T-cell malignancies. We examined the TCL1 gene expression in 21 patients with adult T-cell leukemia (ATL) and 5 cell lines, because ATL is reported to have frequent chromosome 14 band q32 aberrations. However, 20 of the ATL patients and all 5 cell lines lacked any TCL1 expression on northern blot analysis, and TCL1 transcripts were only very faintly detected in the remaining one patient. Expansion of our analysis to include other types of hematopoietic malignancies revealed strong expression of the TCL1 gene in almost all tumor cells of B-cell lineage except myelomas. However, no TCL1 signals were encountered in cells of T-cell or myeloid lineages. In normal human tissues TCL1 was found to be expressed in the spleen, lymph nodes and B-lymphocytes of peripheral blood. These results indicate that TCL1 is not a major target gene for ATL, but that it may play a role in B-cell differentiation and proliferation.