Primary mediastinal large B-cell lymphoma (PMBL) appears to be a distinct clinicopathologic entity among diffuse large B-cell lymphomas (DLBLs). To find molecular alterations associated with this disease, we compared the mRNAs expressed in 3 PMBLs and 3 peripheral DLBLs by differential display-reverse transcription (DDRT) and identified a mRNA specifically expressed in PMBLs. Sequence analysis showed that this mRNA is encoded by the MAL gene, the expression of which was shown to be restricted to the T-cell lineage during hematopoiesis. MAL gene expression was demonstrated by Northern blot and reverse transcription-polymerase chain reaction (RT-PCR) in 8 of 12 PMBLs. However, there was little or no MAL gene expression in 8 peripheral DLBLs. Immunohistochemical analysis evidenced expression of MAL protein in tumoral B cells restricted to the PMBL subtype. Finally, Southern blot studies did not demonstrate rearrangement of the MAL gene. Altogether, our results indicate that MAL expression is recurrent in PMBLs, providing further evidence that PMBL represents a distinct entity among DLBLs. Because MAL protein is located in detergent-insoluble glycolipid-enriched membrane (GEM) domains involved in lymphocyte signal transduction, abnormal expression of MAL protein in the B-lymphoid lineage may have significant implications in PMBL lymphomagenesis.