**1. Introduction**

510 Breast Cancer – Focusing Tumor Microenvironment, Stem Cells and Metastasis

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tumor cells and macrophages is required for tumor cell migration in mammary

JAM-1 in AR42J cells treated with activin A and betacellulin and the diabetic

desmosomes and a ligand for CD11b/CD18-mediated neutrophil transepithelial

junctions is mediated by adhesive interactions between epithelial coxsackie and adenovirus receptor and a junctional adhesion molecule-like protein on Worldwide, breast cancer is the most frequent cancer diagnosed in women and is the second-most leading cause of cancer related deaths in women (Jemal, Bray et al. 2011). Death from breast cancer is most often the result of the spread of the primary tumour to distant sites, where the cancer cells lodge and develop into metastases. Depending on the site of the metastasis, the patient may live for years with reduced quality of life and needing increased health care resources. There is clearly a need for a greater understanding of the molecular events involved in breast cancer metastasis in order to improve treatment options for breast cancer patients and develop therapies aimed at preventing breast cancer metastasis.

Here we will summarize what is known about the molecular basis of breast cancer metastasis and discuss the use of *in vivo* and primarily *in vitro* model systems to study it.
