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Fifty-five cases of primary extranodal high-grade B-cell non-Hodgkin's lymphoma were investigated for bcl-2 and p53 protein expression as well as for t(14;18) translocations and p53 mutations. Phenotypic and genotypic profiles were compared between tumours of gastric (27 cases) and non-gastric (28 cases) origin. bcl-2 protein expression was significantly lower in gastric (11/27) than in non-gastric (28/28) lymphomas (p<0.0001), while nuclear p53 protein expression did not differ significantly between these two groups. In the stomach, there were no significant differences in either bcl-2 or p53 expression profiles between high-grade lymphomas with (n=14) and without (n=13) evidence of a low-grade component of MALT type. However, secondary high-grade lymphomas showed a significant down-regulation of bcl-2 protein (p<0.0001) and, conversely, an up-regulation of p53 protein (p<0.0001) as compared with their low-grade tumour components. In extranodal high-grade B-cell lymphomas, bcl-2 protein expression was not associated with t(14;18) translocation. Only one gastric lymphoma had a p53 point mutation with potential alteration of the amino acid sequence. These findings indicate that primary gastric high-grade B-cell lymphomas are immunohistologically distinct from primary extranodal high-grade B-cell lymphomas of an origin other than in the stomach.