DSS1 is a small， highly acidic protein widely conserved among eukaryotes as a component of the 19S proteasome and implicated in ubiquitin-mediated proteolysis. The BRCA2 tumor suppressor protein functions in homologous recombinational repair (HRR) of DNA double-strand breaks， and does so in part through the actions of a carboxy-proximal region that binds DNA and several other proteins， including DSS1. In the unicellular eukaryote Ustilago maydis， Dss1 interacts with Brh2， a BRCA2-like protein， and regulates its function in mediating HRR. We used RNA interference to deplete DSS1 in human cells， and assayed the effects on double-strand break repair by homologous recombination. Partial depletion of DSS1 protein in human cells reduced the efficiency of HRR to small fractions of normal levels. Residual HRR activity correlated roughly with the residual level of DSS1 expression. The results imply that mammalian DSS1 makes a critical contribution to the function of BRCA2 in mediating HRR， and hence to genomic stability. Activity of the ubiquitin-proteasome system can influence HRR. However， treatment with proteasome inhibitors only partially reproduced the effects of DSS1 depletion on HRR， suggesting that the function of DSS1 in HRR involves more than proteolysis per se.