BRCA1 is a BRCT domain-containing, a tumor-suppressing protein expressed in the cells of breast and other tissues. By targeting the (BRCT)2 domain, BRCA1-IN-1 (Compound 15a) inhibits BRCA1 activities in tumor cells, sensitizes these cells to ionizing radiation-induced apoptosis, and shows the synergistic inhibitory effect when used in combination with Olapari and Etoposide. BRCA1-IN-1 can effectively inhibit HR activity by binding to BRCA1(BRCT)2, and functionally mimic the genetic knockdown of BRCA1. BRCA1-IN-1 is useful in targeting BRCA1/PARP-related pathways involved in DNA damage and repair response, for cancer therapy.