Topoisomerases regulate topological states of DNA. DNA topoisomerase II-binding protein 1 (TopBP1) is a protein that interacts with the C-terminal region of topoisomerase II beta (Topo II beta). TopBP1 function has been found to be important to various aspects of cellular growth control such as DNA replication, the DNA damage checkpoint, and cell survival. TopBP1 interacts with several proteins through its BRCA1 C-terminal (BRCT) motifs. TopBP1 interacts and inhibits E2F1 transcriptional activity through an Rb-independent mechanism that involves recruitment of the SWI/SNF chromatin-remodeling complex proteins Brg1 and Brm.