tumor metastasis including tumor-induced angiogenesis, tumor invasion, and establishment of metastatic foci at the secondary site [46]. We showed by a gelatin zymography assay that the MMP2 activity was down regulated after SOX2 knocked down (Figure 4C), suggesting it is one of the mediators for the SOX2 effect on cell invasion/migration, and tumor metastasis. MMP2 has been shown to be important for metastasis in colorectal cancer [47,48]. Finally, we showed that SOX2 expression correlated to more advanced tumor stages of T3–T4 (Fig. 6G). In addition, we also found that SOX2 expression in primary human colorectal cancer tissues could predict tumor metastasis in human CRC patients. However, we do not know whether SOX2 are expressed in the metastasis tissues because we did not have metastasis tissues for IHC staining. A supporting evidence for a role of SOX2 in metastasis is that we found that the primary cell line SW480 did not express SOX2 but the metastatic cell line SW620 expressed SOX2 (data not shown). The SW480 and SW620 tumor cell lines are respective primary and metastatic colon adenocarcinoma cell lines established from the same patient [49]. In summary, we demonstrated that silencing SOX2 could induce a mesenchymal-epithelial transition in colorectal cancer cells, which partially explain the observation that SOX2 knock down lowered cell mobility and invasion ability in vitro and reduce metastasis in vivo. The molecular events involved in SOX2 silencing mediated phenotypic changes include expected changes in epithelial and mesenchymal markers during EMT, relocation of ß-catenin and reduced WNT pathway activity. However, the fact that we were not able to rescue, reverse or initiate the EMT process after overexpressing SOX2 (either alone or as SOX2-GFP) References 1. 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Cancer Res 66: 8319–8326. PLOS ONE | www.plosone.org 8 August 2012 | Volume 7 | Issue 8 | e41335