The appropriate set of endothelial receptors is sometimes not expressed constitutively and the cancer cells have to trigger their expression. In this context, the culture supernatants of cancer cells can trigger the expression of E- selectin by endothelial cells suggesting that cancer cells may release by themselves cytokines such as TNF-α, IL-1β or INF-γ that will directly activate endothelial cells to express E-selectin, P-selectin, ICAM-2 or VCAM. On the other hand, several studies further show that cancer cells may initiate the expression of endothelial adhesion molecules in a more indirect ways.