A, three-, four-, five- and six-unit stoichiometries. In general, ion channels tend to form a single, centred pore surrounded by symmetrical assembly of three, four, five, or six pore-forming structural units. B, ion channels with multiple channel pores. In some multimeric channels, each monomeric subunit forms a functionally independent channel pore. Left, the dimeric CLC Cl− channels (two pores). Right, the tetrameric aquaporin channels (four pores). Pore-forming structural units are shown with filled grey circles, and channel pores are indicated by open circles with a thick line.