HUS1 is a core component of the 9-1-1 checkpoint complex (RAD9-RAD1-HUS1) that functions as a PCNA-like sliding clamp crucial for DNA damage response and repair 1. The 9-1-1 complex is loaded onto damaged DNA by the RAD17-clamp loader complex and serves as a platform for multiple DNA repair processes 1. HUS1 directly interacts with RAD1 and RAD9 to form a heterotrimeric complex that facilitates ATR-mediated Chk1 phosphorylation, leading to cell cycle checkpoint activation 12. Structurally, the 9-1-1 complex forms a toroidal structure similar to PCNA but with unique interdomain connecting loops that provide damage repair-specific activity 3. The complex stimulates base excision repair by enhancing DNA polymerase beta activity, FEN1 endonuclease cleavage, and DNA ligase I function 4. HUS1 also participates in recruiting RHINO to double-strand breaks during S phase, which directs microhomology-mediated end-joining repair during mitosis 5. The protein plays essential roles in maintaining genomic stability through its involvement in DNA replication checkpoints, homologous recombination repair, and apoptosis regulation 67. A paralog HUS1B exists but cannot functionally substitute for HUS1 in the 9-1-1 complex 8.