**5. Conclusions**

Breast cancer is a complicated disease and progression to metastasis may occur by clonal expansion or parallel progression. Changes in gene expression may occur between the primary tumour and the site of metastasis and development of therapeutics aimed at either the breast or stromal cells at the site of the metastasis will likely be needed to develop better therapeutics against breast cancer metastasis. Some of these new therapeutics may be aimed at reconstituting the expression of breast cancer metastasis inhibitor genes and much research is being done in this field. *In vivo* and *in vitro* model systems have contributed in many ways to our understanding of breast cancer metastasis and will surely continue to do so.
