**2. Breast cancer metastasis**

In the United States, about 178,480 new cases of invasive breast cancer were diagnosed in 2007 and approximately 40,460 women died (Jemal et al, 2007). In 2010, about 207,090 new breast cancer cases were diagnosed and 39,840 died (Jemal et al, 2010). The breast cancer incidence rate has been decreasing in the USA since 1999 and the majority of 40,000 women died each year were due to breast cancer metastasis (Giordano & Hortobagyi, 2003; Jemal et al, 2010). Cancer metastasis is a complex process that includes intercellular and intracellular signaling, activation, adhesion, migration and invasion (Im et al, 2004; Lee et al, 2006). Epithelial-to-mesenchymal transition (EMT) is also thought to be involved in cancer metastasis. EMT may promote cancer-cell progression and invasion into the surrounding microenvironment. Historically, epithelial and mesenchymal cells are distinct in their unique cellular appearance and the morphology of the multicellular structures they create (Shook & Keller, 2003). A typical morphology of epithelium is sheeted and thick with individual epithelial cells abutting each other in a uniform array. Cell-to-cell junctions and adhesions between neighboring epithelial cells hold cells tightly together and inhibit the movement of individual cells away from the epithelial monolayer. Mesenchymal cells, on the other hand, possess usually a more extended and elongated shape and do not exhibit either a regimented structure or tight intracellular adhesion. Mesenchymal cells are irregular in shape and not uniform in composition or density. Adhesions between mesenchymal cells are not as strong as those of their epithelial counterparts, allowing for increased migratory capacity. The transformation of an epithelial cell into a mesenchymal cell not only alters cellular morphology, architecture, adhesion capacity, and migration capacity but also enhances capability of the cell to metastasize (Shook & Keller, 2003). Conversely, the transformation of a mesenchymal cell into an epithelial cell (MET) may prevent cell invasion and suppress cell metastatic ability.
