Two recurrent translocations have been associated with mucosa-associated lymphoid tissue (MALT)-type lymphoma, t(11;18)(q21;q21) and t(1;14)(p22;q32). The first, t(11;18)(q21;q21), results in the fusion protein API2-MLT (API2-MALT1). Through t(1;14)(p22;q32), the BCL10 gene is entirely transferred to the IgH gene, resulting in its overexpression. Wild-type BCL10 is implicated in apoptosis, and it has been suggested that mutated forms gain oncogenic activity. The occurrence of genomic BCL10 mutations in 35 gastric MALT-type lymphomas with or without t(11;18)(q21;q21) (10 and 25 cases, respectively) was investigated. DNA extracted from either whole tissue sections or microdissected clusters of tumor cells was used. Five polymerase chain reactions amplifying the coding exons were performed and were followed by direct sequencing of the products. Twenty differences with the published BCL10 sequence, all single nucleotide substitutions, were detected in 16 cases. Of these, 12 represented known polymorphisms, either at codon 8, 213, or 5. Of the remaining 8 substitutions, 2 were silent and 6 resulted in amino acid substitutions. Mutation analysis results were correlated with the BCL10 expression pattern. Aberrant nuclear BCL10 expression was detected in 14 cases. No association could be demonstrated between the latter and the presence of BCL10 mutations. In contrast, all 10 cases carrying t(11;18)(q21;q21) showed nuclear expression, whereas this staining pattern was absent in 21 of 25 cases without t(11;18)(q21;q21). These results demonstrate that BCL10 mutations are rare in gastric MALT-type lymphoma and are not related to the aberrant nuclear expression of BCL10. In contrast, they indicate that the presence of the API2-MLT fusion protein is associated with aberrant nuclear BCL10 expression.