When using immunofluorescence to define HER2+CTCs, 2 systems have been utilized; HER2+ CTCs defined as cells that express any HER2, or defined using a graded numbering system (0–3), based on HER2 staining intensity at the single cell level, similar to tumor tissue assessment [38,43,51,76]. The gene discussed is ERBB2; the disease is neoplasm.