NIMBUS: Nivolumab Plus Ipilimumab in Metastatic Hypermutated HER2-negative Breast Cancer

Inclusion Criteria: * Participants must have histologically or cytologically confirmed invasive breast cancer, with metastatic disease. Participants without pathologic or cytologic confirmation of metastatic disease should have unequivocal evidence of metastasis from physical examination or radiologic evaluation. * Breast cancer must be HER2-negative by IHC or non-amplified as determined by the current ASCO-CAP criteria. If patient has more than one histological result, the most recent one will be usedfor inclusion. Participants may be ER/PR positive or negative. * Patients must harbor tumors with total mutational burden (TMB) of at least 9 mutations per megabase assessed by a cancer-gene panel containing more than 300 genes, and performed in a CLIA verified laboratory. Tests like Foundation One, Oncopanel (DFCI), or IMPACT (MSKCC) are acceptable for including patients on this trial. * Participants must have measurable disease by RECIST version 1.1. * Participants must agree to undergo a research biopsy, if tumor is safely accessible, at baseline and at day 29 cycle 1 (+14 scheduling window). Previously collected archival tissue will be obtained on all participants. Participants for whom newly-obtained samples cannot be provided (e.g. inaccessible or participant safety concern) may submit an archived specimen (block or if not possible, 20 unstained slides). * Prior chemotherapy: Participants may have received 0-3 prior chemotherapeutic regimens for metastatic breast cancer a…