Patients with non-metastatic HER2+ breast cancer had higher serum miR-21 median levels than patients with non-metastatic HER2− disease (p = 0.044); whereas patients with metastatic HER2+ breast cancer had higher serum miR-10b median levels than patients with metastatic HER2− disease (p = 0.0004). This evidence concerns the gene ERBB2 and breast cancer.