The creation of a functional antibody gene requires the precise recombination of gene segments initially separated on the chromosome. Frequently errors occur in the process, resulting in the formation of a non-functional gene. The non-functional genes can be generated by incomplete rearrangements1, frame-shifts2, or the use of pseudo V or J joining segments3. It is likely that these aberrant rearrangements arise by the same mechanism as is used in generating functional genes, a process which we have suggested may involve unequal sister chromatid exchange 4. Aberrant rearrangements of immunoglobulin genes occur in normal lymphocytes and play a major part in allelic exclusion5. However, it has recently been suggested that aberrant rearrangements involving immunoglobulin and non-immunoglobulin genes may be involved in tumorigenesis. This suggestion has been stimulated by the frequent occurrence of translocations involving chromosomes known to carry immunoglobulin genes in B-cell malignancies. The rearrangement of non-immunoglobulin DNA to the heavy-chain locus has recently been reported6,7. Some aberrant rearrangements of the κ locus appear to be due to rearrangements to sites that do not include the conventional sequence for V gene segment joining8. Here we describe an aberrant κ rearrangement that has led to the joining of DNA from chromosomes 15, 6 and 12, and so appears to be the result of chromosomal translocations or transpositions. As 15/6 or 15/12 translocations have frequently been found in mouse plasmacytomas (as have analogous translocations in human lymphocyte tumours) this aberrant κ rearrangement may be unique to the plasmacytoma from which it was isolated.