An inhibitor of homology-dependent DNA repair; lethal to BRCA2-/- but not BRCA2+/+ PEO1/4 cells (LD50s = 8.7 and >100 μM, respectively); selectively inhibits HDR over NHEJ; decreases the number of U20S cells with ionizing radiation-induced BRCA1 foci but not 53BP1 or pDNA-PK foci; increases the number of ionizing radiation-induced DSBs in DLD-1 cells; cytotoxic to BRCA2-/- DLD-1, ATM-/- GM05849, and PTEN-/- U251 cells from 1-20 μM; delays tumor growth and increases survival in a BRCA2-/-, but not a BRCA+/+, DLD-1 mouse xenograft model at 3 mg/kg ,