Human DNA topoisomerase II binding protein 1 (TopBP1) contains eight BRCT motifs that are found in proteins regulating the DNA damage response, transcription, and replication (1). In addition, TopBP1 shares sequence similarity with the fission yeast Rad4/Cut5 protein and the budding yeast DPB11 protein, both of which are required for DNA damage control and/or replication checkpoint control (2,3). Phosphorylation of TopBP1 occurs in response to DNA double-strand breaks and replication blocks (2). TopBP1 forms nuclear foci and localizes to the sites of DNA damage or the arrested replication forks (2,3). Downregulation of TopBP1 leads to reduced cell survival, probably due to increased apoptosis (2). TopBP1 functions as a transcriptional coactivator by enhancing the human papillomavirus (HPV) transcription/replication factor E2 (1). In addition, the HECT-domain ubiquitin ligase, hHYD, cooperates with TopBP1 in DNA damage response (4). TopBP1 specifically interacts with the C-terminal region of topoisomerase II beta, which suggests a supportive role for TopBP1 in the catalytic reactions of topoisomerase II beta through transient breakages of DNA strands (5). The gene encoding TopBP1 maps to chromosome 3q22.2 (5).