A model for adhesion-induced signal generation by CEACAM1. (A and B) CEACAM1 molecules on free-membrane surfaces are organized as a mixture of monomers (A, left), parallel A-dimers (not depicted), and multimers (B, right). Upon adhesion-mediating, trans-homophilic antiparallel C-dimerization, the N-terminal D1 domains undergo conformational changes, which induce formation of parallel cis–A-dimers by an allosteric mechanism. The formation of ectodomain A-dimers is transduced by the transmembrane domains to the cytoplasmic domains, bringing them together in a specific configuration, thereby altering their binding interactions with intracellular signal molecules.