The LAZ3/BCL6 gene encoding a Zinc-finger nuclear protein is altered in Non-Hodgkin's Lymphomas (NHLs) by translocations, mutations and/or deletions clustered in its 5' non coding region, in a 3.3 Kbp EcoRI fragment which thus defines the Major Translocation Cluster (MTC). In the present study, we describe at the molecular level the deletions found in the MTC of two (NHL) cases using, (i) DNA obtained from a patient (GUI) with a monosomy 3 and three microdeletions of 101, 22, 25 bp in its unique untranslocated 3q27 allele; (ii) a cell line derived from a patient (VAL) carrying a t(3;4) (q27;p11) translocation and a 2.4 Kbp deletion in the untranslocated allele. As the MTC is recurrently subject to alterations, we have cloned and sequenced the murine equivalent of the human MTC and promoter region in an attempt to identify sequences well conserved in mammals that may be thus important for the LAZ3/BCL6 gene regulation. We show that the human and mouse 5' upstream regions of the LAZ3/BCL6 gene although mainly intronic share a particularly high homology of 79% on the overall sequence. Strikingly, the small sequences which are deleted in patient (GUI) are highly conserved (81%, 100% and 92% respectively). Furthermore, they may play a role in the pathogenesis since proteins prepared from B cell lines and HeLa nuclear extracts bind to these sequences in gel retardation assays. Although a large part of this region is intronic, the high conservation of its sequence and the frequency of alterations in NHLs suggest that they are likely to be significant for the regulation of the LAZ3/BCL6 gene.