Chromosomal rearrangement involving the immunoglobulin gene locus, as a result of marked chromosomal instability, is the hallmark of human multiple myeloma (MM) cells. Since Ku80 plays a key role in the non-homologous end-joining (NHEJ) system, we investigated whether Ku80 alteration contributes to this genetic instability by examining its status in 16 MM cell lines. Our study demonstrated a lack of Ku80 alterations at the protein, mRNA and gene level in 15 out of the 16 cell lines. Only the U266 cell line carried a missense mutation of Ser335Leu in one allele of the cDNA. Six marrow samples derived from myeloma patients also did not show any aberrant Ku80 protein, in terms of size. Accordingly, Ku80 alteration is unlikely to be involved in MM, in disagreement with a previous study reporting frequent presence of a 69-kD Ku80 variant (Ku86v) with reduced DNA binding activity in MM cells.