Figure 1.

The roles of CTCs in tumor metastasis. Circulating tumor cells (CTCs) originating from primary or secondary tumors contain important information on the molecular characteristics relevant to tumor progression and therapy. CTCs can appear either as single cells or clusters, which have an increased metastatic potential and a shorter half-life. Their metastatic potential further depends on the specific microenvironment and local vasculature. CTC clusters may be further divided into two main types—homotypic and heterotypic.