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In this study, the joining sequences between chromosomes 14 and 18 on the 14q⁺ chromosomes of a patient with pre-B-cell leukemia and four patients with follicular lymphoma carrying a t(14;18) chromosome translocation were analyzed. In each case, the involved segment of chromosome 18 has recombined with the immunoglobulin heavy-chain joining segment (J_H) on chromosome 14. The sites of the recombination on chromosome 14 are located close to the 5′ end of the involved J_H segment, where the diversity (D) regions are rearranged with the J_H segments in the production of active heavy-chain genes. As extraneous nucleotides (N regions) were observed at joining sites and specific signal-like sequences were detected on chromosome 18 in close proximity to the breakpoints, it is concluded that the t(14;18) chromosome translocation is the result of a mistake during the process of VDJ joining at the pre …