May be a component of the 9-1-1 cell-cycle checkpoint response complex that plays a major role in DNA repair (PubMed:12445383, PubMed:16951081). Promotes DNA double strand break-induced cell cycle arrest and apoptosis, thereby playing a role in genome stability (PubMed:10882129, PubMed:10646593, PubMed:16951081). Also required for telomere length maintenance and germline immortality (PubMed:10646593, PubMed:16951081, PubMed:22547822). May possess 3'->5' double stranded DNA exonuclease activity (By similarity). Exonucleolytic cleavage in the 3'- to 5'-direction to yield nucleoside 5'-phosphates. Probable component of the toroidal 9-1-1 (RAD9-RAD1-HUS1) complex, composed of hpr-9, mrt-2 and hus-1 (PubMed:12445383). Interacts with hus-1 (PubMed:12445383). Might associate with hpr-9 (Probable). RNAi-mediated knockdown disrupts hus-1 localization to the nucleus. Belongs to the Rad1 family.