Epithelial-mesenchymal transition (EMT) is a biological process that drives 
polarized, immotile epithelial cells to undergo multiple biochemical changes to 
acquire a mesenchymal cell phenotype. The characteristic features of EMT are 
cell apolarity, loss of cellular adhesion, reduced expression of E-cadherin and 
increased migratory capacity, as well as invasiveness. EMT is a physiological 
process that is essential for normal embryonic development. Additionally, 
abnormal activation of EMT contributes to some human pathologies such as tissue 
fibrosis, cancer cell invasion and metastasis. In both situations, the basic 
molecular mechanisms are similar, but lead to different effects depending on 
cell type and biological conditions of the environment. TGF-β is a 
multifunctional cytokine that controls proliferation, differentiation and other 
functions in many cell types. It has been found that neoplastic development 
converts TGF-β into an oncogenic cytokine. It activates various molecular 
processes, which are engaged in EMT initiation. All that makes TGF-β a key 
regulator of EMT.  