Gap junctions are formed by a multigene family of polytopic membrane channel proteins, connexins, that have four hydrophobic transmembrane domains and their N and C termini located on the cytoplasmic membrane face. The C-terminal tail plays important roles in channel regulation by pH and phosphorylation. Conserved cysteine residues stabilize the conformation of the extracellular loops that mediate the 'docking' between connexons in the intercellular channel. Over the past year, electron cryocrystallography of two-dimensional crystals of a truncated recombinant alpha 1 (Cx43) has revealed that the transmembrane boundary of the intercellular channel is lined with alpha helices. Furthermore, a ring of alpha helices resides at the interface with the membrane lipids. A three-dimensional analysis based on images recorded from tilted crystals should reveal the location and secondary structure of additional transmembrane domains, as well as provide important structural details about the interactions between connexins within a hemi-channel and connexon-connexon interactions in the extracellular gap.