To clarify the structural basis of the cell adhesion activity of cadherins, we examined the effects of point mutations of well-conserved amino acid residues in the extracellular domain 1 of cadherin-4 (Cdh4) on the adhesion properties by alanine scanning mutagenesis. Mutations of two web-conserved aromatic amino acid residues in the extracellular domain 1 resulted in abnormal processing of Cdh4 molecules and no cell adhesion activity, whereas mutations of the corresponding aromatic amino acids in the extracellular domain 2 did not show these effects, suggesting a role for the two residues in the extracellular domain 1 in the folding and/or intracellular transport processes of Cdh4. Mutations of the amino acid residues suspected to be involved in strand dimer formation resulted in loss or significant decrease in cell adhesion activity. The mutant Cdh4s showed weak concentration at cell-cell adhesion sites and chemical cross-linking suggested that the strand dimer formation was actually impaired in the mutants. These results are consistent with the zipper model, in which the extracellular domain 1 of Cdh4 has intrinsic strand dimer formation activity in addition to adhesion dimer formation activity, both of which are involved in cell adhesion activity. The zipper model, however, needs further improvement to fully account for the present results.