The clinical phenotype and potential treatment strategies for this event have not been reported.<h4>Methods</h4>A retrospective review of 2583 patients receiving trastuzumab and pertuzumab for the treatment of HER2+ breast cancer from 11/23/2011 to 6/21/2021 was performed at Memorial Sloan Kettering Cancer Center (MSKCC). The gene discussed is ERBB2; the disease is breast carcinoma.