Background: BRCA1 and/or BRCA2 mutations confer sensitivity to poly(ADP-ribose) polymerase (PARP) inhibitors (PARPi). In addition to BRCA1/2, alterations of other genes (PALB2, RAD51C….) implicated in homologous recombination repair (HRR) pathways leads to a BRCAness phenotype that is also associated with PARPi sensitivity. Rucaparib, a potent oral PARP-1, -2 and -3 inhibitor, has shown activity in a phase 1 study of patients (pts) with homologous recombination deficient (HRD) breast cancer (Kristeleit, RS. J Clin Oncol 32:5s, 2014 [suppl; abstr 2573]). This single arm, open-label, multicenter phase II study (NCT02505048) is evaluating the efficacy and safety of rucaparib in pts with HER2- metastatic breast cancer (MBC) associated with a BRCAness phenotype determined by “high tumor genomic LOH” score and/or a somatic BRCAmutation. Methods: Pts with HER2- MBC exhibiting a BRCAness phenotype will receive oral rucaparib 600 mg BID continuously in 21-day cycles until disease progression. The primary endpoint is clinical benefit rate (CBR), defined by complete and partial response and stable disease lasting for at least 16 weeks and, if CBR is significant, the objective response rate (ORR). Secondary endpoints include progression-free survival, overall survival, safety, and the prognostic value of the BRCAness signature. Targeted enrollment is 41 pts using a Simon two-stage design. Eligibility: Women with HER2- MBC with a BRCAness phenotype who received 1-4 prior chemotherapy regimens are eligible. ECOG PS 0-1 and adequate organ function is required. The BRCAness phenotype is defined by high tumor genomic LOH (LOH cutoff of 18%) that can identify HRD tumors, including both known BRCA1 methylation and unknown genetic/epigenetic mechanisms and somatic BRCA1/2 mutations. Pts with a known BRCA1 and/or BRCA2 germline mutation are excluded. “high tumor genomic LOH” score will be generated from the CytoScan HD SNP array, which is available from the SAFIR02 protocol or other molecular programs. To date, 13 pts have been enrolled, with enrollment ongoing. This trial design is intended to establish proof-of-concept that rucaparib can improve ORR in HER2- MBC with HRD. Clinical trial information: NCT02505048