Antiparallel NCAM interactions involve trans-interactions of NCAM molecules on opposed cell membranes. Based on structural and functional studies a 'double zipper' model has been proposed to describe these interactions. The first model - the 'flat zipper'- formed between NCAM1 cis-dimers from one cell surface interacting in trans through IgII-IgIII contacts with NCAM1 cis-dimers from another cell surface. The second model - the 'compact zipper'- is formed between NCAM1 cis-dimers from one cell surface interacting in trans through IgI-IgIII and IgII-IgII contacts with cis-dimers from another cell surface.

Abrogation of cis-dimerization inhibits NCAM mediated neurite outgrowth, and cis-dimerization of NCAM1 may be a necessary prerequisite for subsequent trans-interactions.