Hodgkin-Reed-Sternberg (H-RS) cells are clonal B cells carrying Ig gene rearrangements. However, in situ hybridization methods failed to demonstrate Ig gene expression in H-RS cells of classical Hodgkin's disease (HD). Because somatic mutations rendering potentially functional Ig gene rearrangements nonfunctional were detected in some cases of the disease, it was speculated that H-RS cells in classical HD may have lost the ability to express antigen receptor as a rule. Recently, we established a novel cell line (L1236) from H-RS cells of a patient with mixed cellularity subtype of HD. L1236 cells harbor a potentially functional VH1 and a potentially functional Vkappa3 gene rearrangement. However, no antibody expression was detected. To show potential reasons for this lack of Ig expression, we analyzed the genomic organization of the Ig genes and their transcription in the primary and cultivated H-RS cells of this patient. The H-RS cells were found to have switched their isotype to IgG4, confirming their mature B-cell nature. By amplifying cDNA from L1236 cells as well as from frozen biopsy material transcripts of the Vkappa3 and the VH1 gene rearrangement were detected for both sources of cDNA. However, Northern blot hybridization of L1236 RNA failed to demonstrate VH1 and Vkappa3 transcripts, indicating only a low level of transcription. Sequence analysis of the promoter and leader regions of the VH1 gene rearrangement from L1236 cells as well as from lymphoma-affected tissue showed a somatic mutation in the conserved octamer motif of the promoter region. Somatic mutations were also detected within the 3' splice site of the leader intron and adjacent nucleotides in the rearranged Vkappa light chain gene, leading to aberrant splicing. These mutations might prevent the generation of adequate amounts of functional Ig gene transcripts as template for translation into protein. Thus, mutations in H-RS cells that prevent Ig gene expression might also be located outside the coding region of the Ig genes.