DNA topoisomerase 2-binding protein 1 (TOPBP1) is a scaffold protein that in humans is encoded by the TOPBP1 gene. TOPBP1 was first identified as a protein binding partner of DNA topoisomerase-IIβ by a yeast 2-hybrid screen, giving it its name. TOPBP1 is involved in a variety of nuclear specific events. These include DNA damage repair, DNA replication, transcriptional regulation, and cell cycle checkpoint activation. TOPBP1 primarily regulates the DNA damage repair response through its ability to activate the damage response kinase, ataxia-telangiectasia mutated and RAD3-related (ATR). It also plays a critical role in DNA replication initiation and regulation of the cell cycle. Changes in TOPBP1 gene expression are associated with pulmonary hypertension, breast cancer, glioblastoma, non-small cell lung cancer, and sarcomas.