HUS1 is a component of an evolutionarily conserved, genotoxin-activated checkpoint complex. This protein associates with Rad9 and Rad1 to form the 9-1-1(RAD9-RAD1-HUS1) complex, which localizes to DNA lesions and promotes DNA damage signaling and repair or apoptosis, cell cycle arrest. The trimeric complex is structurally similar to the proliferating cell nuclear antigen (PCNA) sliding clamp and interacts with Rad17 as a clamp-clamp loader pair during the DNA damage response.