It is worth noting that many recent studies have shown that CTCs are heterogeneous, including epithelial tumor cells, epithelial–mesenchymal transition (EMT) cells, mixed (epithelial and EMT positive) tumor cells, circulating cancer stem cells (CTSC), and irreversible EMT-positive tumor cells; in addition, the expression of CTC surface protein epithelial cell adhesion molecule (EpCAM) is dynamic. Here, EPCAM is linked to cancer.