CBP-93872 was previously identified as a G₂ checkpoint inhibitor using a cell-based high-throughput screening system. However, its molecular actions as well as cellular targets are largely unknown. Here, we uncovered the molecular mechanisms underlying abrogation of the G₂ checkpoint by CBP-93872. CBP-93872 specifically abrogates the DNA double-stranded break (DSB)–induced G₂ checkpoint through inhibiting maintenance but not initiation of G₂ arrest because of specific inhibition of DSB-dependent ATR activation. Hence, ATR-dependent phosphorylation of Nbs1 and replication protein A 2 upon DSB was strongly suppressed in the presence of CBP-93872. CBP-93872 did not seem to inhibit DNA-end resection, but did inhibit Nbs1-dependent and ssDNA-induced ATR activation in vitro in a dose-dependent manner. Taken together, our results suggest that CBP-93872 is an inhibitor of maintenance of the DSB-specific G₂ checkpoint and thus might be a strong candidate as the basis for a drug that specifically sensitizes p53-mutated cancer cells to DSB-inducing DNA damage therapy. Cancer Res; 74(14); 3880–9. ©2014 AACR.