Abstract
Metastatic breast cancer (MBC) is the most life-threatening disease in women worldwide. HER2-mutated breast carcinoma has been reported to benefit from HER2-targeted tyrosine kinase inhibitors recently. Here, we presented a heavy pretreated and harbored HER2 V777L mutation de novo stage IV Luminal B (HER2 unamplified) breast cancer patient who achieved an unexpected good response to trastuzumab combined with vinorelbine therapy. Although HER2-unamplified MBC patients do not regularly benefit from anti-HER2 target therapy, HER2 V777L mutation detected by next-generation sequencing from ctDNA may present as a predictive biomarker for anti-HER2-based strategy therapy in HER2-negative MBC patients.