Cytokine biomarkers of breast CTCs used in clinical detection include easily accessible membrane-localized biomarkers such as EpCAM/CD326 for epithelial CTCs in early metastasis, RGD-peptide-enriched vimentin and fibronectin for mesenchymal CTCs in advanced metastasis, CK-8/18/19 as general non-native blood cell marker, and CA15-3 as a breast-cancer-specific surface marker [139,140,141,142]. The gene discussed is EPCAM; the disease is breast cancer.