3.4. CTCs and tumor initiating cellsFurther characterization of CTCs identified subpopulations such as tumor initiating cells (TICs). TICs, also called cancer stem cells (CSCs), are likely to be critical not only for diagnosis and treatment of early stage cancer but also for finding novel treatments for pancreatic cancer with advanced stage. Numerous reports have demonstrated that EpCAM is one of the major markers for TICs of various cancers including pancreatic cancer [33]. Li et al. identified a subpopulation of highly tumorigenic cancer cells expressing the cell surface markers CD44+, CD24+ and EpCAM+ in pancreatic cancer. These cells display stem cell-like characteristics such as self-renewal and the ability to differentiate as well as to drive continuous malignant cell expansion in an invasive and metastatic manner. As few as one hundred CD44+CD24+EpCAM+cells were able to generate a tumor in 50% of immunocompromised mice. In contrast, cells which were negative for all three markers required at least 104 or more tumorigenic cells to be implanted in order to induce tumor formation [34].Theoretically, there are two subtypes of cancer stem cell pools within a tumor: intrinsic cancer stem cells and induced cancer stem cells. Intrinsic cancer stem cells are thought to exist within the primary tumors from the initial stage of tumorigenesis whereas the induced cancer stem cells are differentiated cancer cells which have undergone epithelial mesenchymal transition (EMT). Epithelial mesenchymal transition results from genetic and epigenetic changes of cancer cells induced by transformation signals released from their microenvironment and from stroma cells [35]. Conventionally, EMT is recognized as a pathological mechanism during the progression of various diseases including inflammation, fibrosis and cancer [36]. During the EMT process, epithelial cells undergo multiple biochemical changes involving down-regulation of epithelial markers, which confer cell-to-cell and cell-to-extracellular matrix (ECM) adhesion. There is also up-regulation of mesenchymal markers in epithelial cells during the EMT process, conferring increased production of ECM components, enhanced migratory and invasive capacity, and increased resistance to apoptosis [37], [38]. New tumor initiation at the metastatic site is now believed to originate from cells which were first transformed from an epithelial to a mesenchymal state and then migrated to the site of the metastasis. Once that cell has moved to a new location, it can undergo mesenchymal to epithelial transition (MET), the reverse procedure [39]. A recent study conducted on the CTCs from metastatic breast cancer patients showed that a major proportion of CTCs were positive for both EMT markers and stem cell characteristics, thus providing further evidence of the link between cancer stem cells and CTCs [40].