Metastasis formation involves several steps. Two possible ways of metastasis formation: (a) first, single tumor cells disseminate form the primary tumor and invade the extracellular matrix, intravasate into blood or lymph vessels, and get transported and adhere to the endothelium. Secondly, (a) invasive tumor cells grow in vessels and do not transmigrate or (b) invasive tumor cells transmigrate through the endothelium, invade and from a secondary tumor in the targeted tissue.