Cell migration plays roles in invasion of transformed cells and scattering of embryonic mesenchymal cells into surrounding tissues. We have found that Ig-like Necl-5/Tage4 is up-regulated in NIH3T3 cells transformed by an oncogenic Ras (V12Ras-NIH3T3 cells) and heterophilically trans-interacts with a Ca 2؉ -independent Ig-like cell adhesion molecule nectin-3, eventually enhancing their intercellular motility. We show here that Necl-5 furthermore enhances cell migration in a nectin-3-independent manner. Studies using L fibroblasts expressing various mutants of Necl-5, NIH3T3 cells, and V12Ras-NIH3T3 cells have revealed that Necl-5 enhances serum-and platelet-derived growth factor-induced cell migration. The extracellular region of Necl-5 is necessary for directional cell migration, but not for random cell motility. The cytoplasmic region of Necl-5 is necessary for both directional and random cell movement. Necl-5 colocalizes with integrin ␣ V  3 at leading edges of migrating cells. Analyses using an inhibitor or an activator of integrin ␣ V  3 or a dominant negative mutant of Necl-5 have shown the functional association of Necl-5 with integrin ␣ V  3 in cell motility. Cdc42 and Rac small G proteins are activated by the action of Necl-5 and required for the serum-induced, Necl-5-enhanced cell motility. These results indicate that Necl-5 regulates serum-and platelet-derived growth factor-induced cell migration in an integrin-dependent, nectin-3-independent manner, when cells do not contact other cells. We furthermore show here that enhanced motility and metastasis of V12Ras-NIH3T3 cells are at least partly the result of up-regulated Necl-5.In multicellular organisms, cell migration is essential for normal development and responses to tissue damages and infection throughout life (1, 2). Cell migration is also observed in many diseases, such as cancer and atherosclerosis (3, 4). Cells migrate as individuals or as groups; leukocytes, lymphocytes, and fibroblasts migrate as individuals, whereas epithelial and endothelial cells migrate as groups. Cell migration is divided into at least four mechanistically separate steps: extension of protrusions, formation of new cell-matrix adhesions, contraction of cell body, and tail detachment (1, 5). Cell migration is normally directed and controlled by extracellular cues, such as growth factors, cytokines, and extracellular matrix molecules. These cues stimulate cell surface receptors to initiate intracellular signaling through second messengers, protein kinases, protein phosphatases, and heterotrimeric large and monomeric small G proteins to regulate the multiple steps. When migrating cells contact other cells, they stop migration and proliferation (6, 7). This phenomenon is known for a long time as contact inhibition of cell movement and proliferation. Transformation of cells causes disruption of cell-cell adhesion, increase of cell motility, and loss of contact inhibition of cell movement and proliferation, eventually leading transformed cells to invasion into surrounding t...