TOPBP1 (DNA topoisomerase II binding protein 1) is a scaffold protein that serves as a critical adapter in DNA replication initiation and DNA repair processes 1. The protein contains multiple BRCT domains that recognize and bind phosphorylated proteins, enabling it to bring together functional protein combinations during DNA damage responses 2. TOPBP1 plays essential roles in multiple DNA repair pathways: it facilitates homologous recombination by promoting RAD51 loading and RPA displacement during S-phase 3, and activates microhomology-mediated end-joining (MMEJ) repair during mitosis by recruiting PLK1-phosphorylated polymerase theta (POLQ) to DNA damage sites 2. The protein is crucial for replication initiation, being required for loading replication factors including CDC45 and GINS complex components onto chr3 4. In mitosis, TOPBP1 forms complexes with CIP2A to maintain chr3 stability by tethering broken chr3 and preventing mis-segregation of acentric chromosome 3 15. TOPBP1 also activates ATR kinase signaling and checkpoint responses to DNA damage 6. Clinically, the CIP2A-TOPBP1 axis represents a synthetic lethal target for BRCA-mutated cancers, as these cells depend on this complex for survival when homologous recombination is deficient 1. The protein's multifaceted roles make it central to genome stability maintenance across different cell cycle phases.