t(9;14)(p13;q32) is a rare but recurring translocation found in a subset of B-cell non-Hodgkin's lymphoma (B-NHL). These lymphomas share clinical features with chronic lymphocytic leukemia and are further characterized by plasmacytoid differentiation of lymphoma cells. Molecular cloning of t(9;14)(p13;q32) revealed juxtaposition of the PAX5 to the immunoglobulin heavy chain gene (IGH), although breakpoints on both genes were variable. The PAX5 gene encodes the BSAP (B-cell-specific activator protein) transcription factor, which is expressed throughout the process of B-cell development except in terminally differentiated plasma cells. t(9;14)(p13;q32) consistently leaves the PAX5 coding region intact, most likely resulting in deregulated expression of the gene product due to the proximity of IGH. The majority cases of B-cell tumors expressed considerable levels of PAX5/BSAP irrespective of whether they exhibited t(9;14)(p13;q32), suggesting that quantitative differences in expression level alone may not account for the development of this particular subtype of B-NHL.