Taken together, alpha 4 integrins may influence metastatic process at various stages (Fig. 1). The detachment of tumor cells from the primary tumor and the invasion of the surrounding tissue represent the onset of tumor metastasis. There is good experimental evidence that at the primary tumor site expression of alpha 4 integrins inhibits the ability of melanoma cells to break loose. This could be achieved either by strengthening of homotypic adhesion to adjacent tumor cells or by down regulation of matrix metalloproteases that are required for tumor cell migration through the extracellular matrix. After entering the blood circulation, alpha 4 integrins on tumor cells derived from melanomas, sarcomas or lymphomas rather promote than inhibit accumulation of disseminated cells in distant organs. The positive effects of alpha 4 integrins at this stage of metastasis formation appear to depend on alpha 4 integrin interactions with ligands expressed on the surface of endothelial cells. While VCAM-1 is expressed on endothelial cells exposed to inflammatory cytokines, MAdCAM-1 is constitutively expressed on mucosal endothelium. In addition, it is conceivable that tumor cell aggregates trapped in the microcirculation may trigger local inflammatory reactions that result in VCAM-1 up-regulation. Tumor cell-bound alpha 4 integrins may strengthen adhesion to endothelium and promote trans-endothelial migration (HAUZENBERGER et al. 1997; MEERSCHAERT and FURIE 1994). Successful formation of new tumor colonies in distant organs is the final step in the metastatic cascade. Interestingly, alpha 4 integrin dependent mechanisms may either promote or inhibit this process. Thus, it was observed that alpha 4 integrins may direct cancer cells like CHO and lymphoma cells to organ compartments, where ligands for alpha 4 integrins are expressed (e.g., bone marrow). Depending on the tumor type this event may result in enhanced metastasis formation. However, as was documented for murine lymphoma cells alpha 4 integrins may also inhibit tumor cell growth either by inducing apoptosis or by reducing the proliferation rate. Based on numerous studies on human cancers and experimental tumor models, alpha 4 integrins may represent attractive target molecules for therapeutic manipulation of tumor cell behavior. To this end, however, it will be of great importance to precisely define the molecular basis for the adverse effects of alpha 4 integrins on metastasis formation.