Most cell types express several different claudins, which assemble into the same adhesive strands, yet the generality of heterotypic binding across the intercellular space remains an unresolved question. There is limited evidence that a subset of claudins can interact heterotypically across cell contacts; for example, claudin-1 can bind claudin-3 but not claudin-2 (Furuse et al. 1999), but there is currently no information that allows generalizations about which claudins can interact or if adhesion involves additional proteins (Fig. 2A,B). Some claudins are observed to coimmunoprecipitate with a list of proteins (e.g., claudin-7 with EpCAM [Le Naour and Zoller 2008]; claudin-11 with b1 integrin and tetraspanins [Tiwari-Woodruff et al. 2001]), although the biologic implications remain unknown. Further understanding of how various functions (selectivity, adhesion, and assembly) are organized within the protein is severely limited by a current lack of structural information.