Mantle cell lymphoma (MCL) is characterized by the t(11;14) translocation that juxtaposes the bcl-1 locus to immunoglobulin (Ig) gene sequences and leads to deregulation of cyclin D1 gene. t(11;14) is thought to result from an error of the recombinase during D-JH Ig gene assembly; however, data on the underlying mechanism and candidate recombination-targeting motifs in the major translocation cluster (MTC) of the bcl-1 gene are lacking. bcl-1/IgH junctional sequences from seven MCL patients were amplified by PCR using primers targeting MTC and JH sequences on chromosomes 11q13 and 14q32, respectively. PCR products were directly sequenced and junctional sequences were searched for homology to known germline D genes. bcl-t MTC breakpoints were searched for the presence of possible recombination target motifs; heptamers, nonamers, binding sequence of the bp45 nuclease, x-like sequences and D gene segments. bcl-1/JH junctions were found to bear homology to D gene segments (DLR3, DM and DIR5) in 3/7 MCL samples. A computer-based search in previously published and/or submitted to GenBank bcl-1/JH junctional sequences identified homology to D genes in 1/4 MCL tumour samples and 1/4 MCl cell lines; DXP4 or D23/7 and DHQ52 or D22/21 or DXP5, respectively. The MTC locus contained motifs with homology to bp45 nuclease binding sequence, x-like sequences, heptamers/nonamers, D-like DIR genes and non-homologous recombination short (6 bp) DNA sequences. The above data indicate that the t(11;14) translocation in MCL may also occur at a more mature stage of B-cell ontogeny than previously thought, i.e. during VH-to-DJH rearrangement. Various known recombination motifs within MTC may contribute to an illegitimate recombination event between bcl-1 and DJH.