S100A9 and breast cancer: According to current findings, metastatic CTCs in BC have the HER2+EGFR+HPSE+Notch1+ phenotype, co-express surface markers EpCAM, CD44, CD47, MET and are characterized by overexpression of the molecules associated with metastasis in general: TMPRSS4, MUC16, S100A4, S100A9, and NPTN.