Other blood BC biomarkers include proteins that can help assess disease progression, predict recurrence, and facilitate the monitoring of treatment response, such as carcinoembryonic antigen (CEA), gene products of MUC1 (e.g., cancer antigen (CA) 15-3 and CA 27.29), circulating cytokeratins, and serum HER2 [104,105]. The gene discussed is ERBB2; the disease is breast cancer.