Cadmium (Cd) is a toxic heavy metal that is particularly damaging to proximal tubular cells of the kidney. Cd-induced renal toxicity is associated with perturbation of the ubiquitin proteasome system. Our previous study demonstrated that Cd increased gene expression of ubiquitin-coding genes, UBB, UBC, and UBA80. Notably, knockdown of the polyubiquitin gene UBB by siRNA transfection significantly decreased ubiquitinated protein levels that had been increased by Cd treatment. The present study showed that in contrast to UBB, knockdown of the monoubiquitin gene UBA80 did not diminish the Cd-induced protein ubiquitination in HK-2 cells. Taken together, our results suggest that polyubiquitin rather than monoubiquitin is preferably engaged in Cd-induced accumulation of ubquitinated proteins in HK-2 cells.