In this open label, single arm Phase II study of single agent pembrolizumab in metastatic HER2-receptor negative breast cancer patients with germline deletion in A3B...Complete response (CR) was observed in one patient, while partial response (PR) was observed in 4 patients, with an ORR of 22.7% (5 over 22 subjects) in stage I, meeting the pre-defined criteria to proceed to stage II. Notably, the patient with complete response had received 2 prior lines of chemotherapy, whereas of the patients with partial response, 1 had received 1 prior line and 3 had received 3 prior lines of chemotherapy in a metastatic setting. As the observed ORR was greater than the value of r1 (13.6%)...Single agent pembrolizumab demonstrates promising efficacy in germline A3Bdel carriers...