It is thought that the concentration of classic cadherins at cell-cell adhesion sites is essential for generating strong cell-cell adhesion activity, but the mechanism is not well understood. To clarify the structural basis of the concentration process and the cell adhesion activity, we constructed various mutants of cadherin-4 and examined the adhesion properties of the transfectants. A deletion mutant lacking the entire cytoplasmic domain had weak, but significant Ca2+- dependent cell adhesion activity. Interestingly, the deletion mutant showed intrinsic cluster formation in the absence of cell-cell adhesion, possible lateral cluster formation. The cytoplasmic domain deleted cadherin-4 containing the mutation of Trp-a to Ala, which is known to inhibit the strand dimer formation required for the cell-cell adhesion, retained the possible activity of lateral cluster formation, supporting this notion. These results suggest that the extracellular domain has intrinsic activity of lateral cluster formation. Indeed, deletion of a cadherin repeat in the extracellular domain significantly reduced or abolished the lateral cluster formation as well as the concentration of cadherin-Li at cell-cell contact sites and cell adhesion activity, When transfectants of the cytoplasmic domain-deleted cadherin-4 made cell-cell contact and formed intimate cell-cell adhesion, the lateral clusters of cadherin-4 initially gathered at cell-cell contact sites, and a smooth linear concentration was gradually formed along the cell-cell adhesion interface. The results suggest that the lateral cluster formation is involved in the concentration process of cadherin-l at cell-cell adhesion sites, hence in the strong cell adhesion activity of cadherin-4 as well.