The epithelial-to-mesenchymal transition (EMT) in tumor progression: an opportunity to target metastatic tumor cells. The epithelial-to-mesenchymal transition (EMT) and its reverse process, designated as mesenchymal-to-epithelial transition (MET), are involved in the progression of epithelial tumors towards metastasis. Vaccine strategies targeting molecules that control the EMT process, for example, the transcription factor Brachyury, could be used to block tumor spreading.