Involved in microhomology-mediated end-joining (MMEJ) DNA repair by promoting recruitment of polymerase theta (POLQ) to DNA damage sites during mitosis (PubMed:37440612). MMEJ is an alternative non-homologous end-joining (NHEJ) machinery that takes place during mitosis to repair double-strand breaks in DNA that originate in S-phase (PubMed:37440612). Accumulates in M-phase; following phosphorylation by PLK1, interacts with POLQ, enabling its recruitment to double-strand breaks for subsequent repair (PubMed:37440612). Also involved in the DNA damage response (DDR) signaling in response to genotoxic stresses such as ionizing radiation (IR) during the S phase (PubMed:21659603, PubMed:25602520). Recruited to sites of DNA damage through interaction with the 9-1-1 cell-cycle checkpoint response complex and TOPBP1 in a ATR-dependent manner (PubMed:21659603, PubMed:25602520). Required for the progression of the G1 to S phase transition (PubMed:21659603). Plays a role in the stimulation of CHEK1 phosphorylation (PubMed:21659603). {ECO:0000269|PubMed:21659603, ECO:0000269|PubMed:25602520, ECO:0000269|PubMed:37440612}. This is the function of RHNO1 (RAD9-HUS1-RAD1 interacting nuclear orphan 1, Ensembl gene identifier ENSG00000171792).