RAD1 is a checkpoint DNA exonuclease and core component of the 9-1-1 cell-cycle checkpoint response complex, essential for DNA damage recognition and repair 12. The 9-1-1 complex is recruited to DNA lesions by the RAD17-RFC clamp loader and functions as a sliding clamp platform facilitating long-patch base excision repair (LP-BER) 34. RAD1 stabilizes DNA polymerase beta, endonuclease FEN1, and DNA ligase I activity at LP-BER sites, enhancing their catalytic efficiency 564. The 9-1-1 complex also recruits RHINO to double-strand breaks during S phase 7 and directs microhomology-mediated end-joining (MMEJ) repair in mitosis 8. Structurally, RAD1's interdomain connecting loop binds FEN1 peptides with high affinity, providing molecular specificity for damage repair versus replication functions 9. RAD1 is evolutionarily conserved across eukaryotes, with the human locus localized to chromosome 5.2, a region frequently altered in lung and bladder cancers 1011. These findings establish RAD1 as a critical hub coordinating multiple DNA repair pathways and cell cycle checkpoints.