Nectin is a Ca(2+)-independent immunoglobulin (Ig)-like cell-cell adhesion molecule that forms cell-cell adherens junctions cooperatively with E-cadherin in a variety of cells. Nectin has one transmembrane segment and three Ig-like loops in the extracellular region. The first Ig-like loop is essential for the trans-dimer formation of nectin of two neighboring cells, causing cell-cell adhesion. We show here that the second Ig-like loop is essential for the cis-dimer formation of nectin on the same cell, and that the cis-dimer formation is essential for the trans-dimer formation.