Anion channel structures. A) The cystic fibrosis transmembrane conductance regulator CFTR is composed of two repeats of six TMD linked by a regulatory subunit (R) and a nucleotide binding fold (NBF1). A second nucleotide binding fold (NBF2) is found towards the COOH-terminus of the protein. The predicted secondary structures of B) the voltage-gated ClC chloride channels and C) the bestrophin family are still the subject of some controversy, e.g. the precise number of transmembrane domains in the 9...12 region of the ClC channels is not known.