Component of the 9-1-1 cell-cycle checkpoint response complex that plays a major role in DNA repair (PubMed:10846170, PubMed:10884395). The 9-1-1 complex is recruited to DNA lesion upon damage by the RAD17-replication factor C (RFC) clamp loader complex (PubMed:12578958). Acts then as a sliding clamp platform on DNA for several proteins involved in long-patch base excision repair (LP-BER) (PubMed:15871698). The 9-1-1 complex stimulates DNA polymerase beta (POLB) activity by increasing its affinity for the 3'-OH end of the primer-template and stabilizes POLB to those sites where LP-BER proceeds; endonuclease FEN1 cleavage activity on substrates with double, nick, or gap flaps of distinct sequences and lengths; and DNA ligase I (LIG1) on long-patch base excision repair substrates (PubMed:15314187, PubMed:15556996, PubMed:15871698). The 9-1-1 complex is necessary for the recruitment of RHNO1 to sites of double-stranded breaks (DSB) occurring during the S phase (PubMed:21659603). Isoform 1 possesses 3'->5' double stranded DNA exonuclease activity (PubMed:9660799). Exonucleolytic cleavage in the 3'- to 5'-direction to yield nucleoside 5'-phosphates. Component of the toroidal 9-1-1 (RAD9-RAD1-HUS1) complex, composed of RAD9A, RAD1 and HUS1 (PubMed:10846170, PubMed:10884395). The 9-1-1 complex associates with LIG1, POLB, FEN1, RAD17, HDAC1, RPA1 and RPA2 (PubMed:10846170, PubMed:10884395, PubMed:15314187, PubMed:15556996, PubMed:15871698, PubMed:15897895, PubMed:16216273). The 9-1-1 complex associates with the RAD17-RFC complex (PubMed:12578958). RAD1 interacts with POLB, FEN1, HUS1, HUS1B, RAD9A and RAD9B (PubMed:10359610, PubMed:10777662, PubMed:11944979, PubMed:14500360, PubMed:14611806, PubMed:15314187, PubMed:15556996, PubMed:16216273). Interacts with DNAJC7 (PubMed:11573955). Expressed in testis, uterus, bladder, spleen, ovaries, lung, brain and muscle (at protein level). May be produced at very low levels due to a premature stop codon in the mRNA, leading to nonsense-mediated mRNA decay. Belongs to the rad1 family. Truncated N-terminus. Truncated N-terminus.