The BCL10 gene was recently isolated from the breakpoint region of t(1;14)(p22;q32) in mucosa-associated lymphoid tissue (MALT) lymphomas. Somatic mutations of BCL10 were found in not only t(1;14)-bearing MALT lymphomas, but also a wide range of other tumors. To clarify the actual frequency and spectrum of BCL10 mutations in primary B-cell non-Hodgkin's lymphomas (NHL), we examined a total of 139 NHL cases comprising 25 with MALT lymphomas, 54 with follicular B-cell lymphomas (FCL), and 60 with diffuse large B-cell lymphomas (DLBL). Polymerase chain reaction single-strand conformation polymorphism (PCR-SSCP) and sequencing analyses led to the identification of four nucleotide changes in FCL and one in DLBL. In contrast, no BCL10 mutations were found in our series of MALT lymphomas. While screening for mutations, we also found three polymorphic sequence variants at codons 5 and 213 and in intron 1 of the BCL10 gene. Our results strongly suggest that somatic mutations of BCL10, if they occur at all, are rare in B-cell NHLs and do not commonly contribute to their molecular pathogenesis.