The serum markers used most widely to predict clinical response to trastuzumab-based anti-HER2 therapy (i. e. trastuzumab alone or associated with adjuvant chemotherapy, such as paclitaxel) in patients with HER2-positive metastatic breast cancer are cancer antigen 15.3 (CA15.3) and carcinoembryonic antigen (CEA). The gene discussed is ERBB2; the disease is cancer.