Thirty to forty percent of diffuse large B cell lymphomas (DLBCL) carry BCL-6 translocations that disrupt its 5' regulatory region. This same region is also subject to somatic hypermutations, although only a small fraction of these mutations have a detectable effect on transcription. Here, we show that transcription of the BCL-6 gene is negatively self-regulated in multiple cell types. This mechanism operates by means of the interaction of two BCL-6-binding sites within exon 1 of the gene and the BCL-6 protein itself, which is a potent transcription repressor. Because the DLBCL-associated "activating mutations" specifically target these exon 1 binding sites, and because the entire exon 1 is usually removed in the BCL-6-translocated tumors, this autoregulation is bypassed in 30-40% of all DLBCL cases. Our results not only demonstrate an important mechanism governing the expression of BCL-6, but also explain how BCL-6 is deregulated in a large number of DLBCL patients, providing a better understanding of BCL-6-related lymphomagenesis.