The metastatic steps are critically regulated by various cell types, including the blood vessel cells (endothelial cells), immune cells or stromal cells. The growth of a new network of blood vessels, called tumor angiogenesis, is a crucial hallmark of cancer. It has therefore been suggested that angiogenesis inhibitors would prevent the growth of metastases. Endothelial progenitor cells have been shown to have a strong influence on metastasis and angiogenesis. Endothelial progenitor cells are important in tumor growth, angiogenesis and metastasis, and can be marked using the Inhibitor of DNA Binding 1 (ID1). This novel finding meant that investigators gained the ability to track endothelial progenitor cells from the bone marrow to the blood to the tumor-stroma and even incorporated in tumor vasculature. Endothelial progenitor cells incorporated in tumor vasculature suggests that this cell type in blood-vessel development is important in a tumor setting and metastasis. Furthermore, ablation of the endothelial progenitor cells in the bone marrow can lead to a significant decrease in tumor growth and vasculature development. Therefore, endothelial progenitor cells are important in tumor biology and present novel therapeutic targets. The immune system is typically deregulated in cancer and affects many stages of tumor progression, including metastasis.