ERBB2 and breast neoplasm: On the basis of the difference of HER2 expression on single CTCs between patients with or without HER2+ breast tumors (HER2+ vs. non-HER2+), we defined four subpopulations of CTCs with various MFI ranges of HER2 immunostaining, including HER2(0) CTCs (MFI 0—100 a.u.), HER2(1+) CTCs (MFI 100—400 a.u.), HER2(2+) CTCs (MFI 400—600 a.u.)and HER2(3+) CTCs (MFI >600 a.u.)(Figure 3).