Figure 1: Involvement of EMT for metastasis progression. In the primary tumor, EMT-derived cells (red) acquire a mesenchymal-like migratory phenotype during tumor progression. They lose their epithelial properties, enhance invasive capacities and enter the bloodstream, contributing to the generation of CTC. These specific CTC are proposed to extravasate and grow in distant organs, thereby forming metastases. After that, a reverse EMT process (mesenchymal-to-epithelial transition, MET) appears to occur, leading to a redifferentiation of tumor cells to form micrometastases. Meanwhile, some CTC (blue), which are actually detected by current CTC technologies, are not able to undergo EMT, owning to the lack of cancer stem cell properties, but those cells can also disseminate through the bloodstream into distant organs without forming micrometastases.