Since increased levels of miR-19a-3p in the serum of patients with metastatic HER2 + breast cancer treated with trastuzumab were associated with favorable clinical outcomes, we believe that miR-19a-3p could serve as a circulating blood biomarker to identify patients with effective anti-tumor immune responses activated by trastuzumab, which involves NK cell-mediated ADCC and the activation of CD4 + Th1 cells. Here, CD4 is linked to breast cancer.