Thomsen disease is associated with dominant mutations and Becker disease with recessive mutations in CLCN1. Members of Epithelial Chloride Channel (E-ClC) Family (TC# 1.A.13) catalyze bidirectional transport of chloride ions. Mammals have multiple isoforms (at least 6 different gene products plus splice variants) of epithelial chloride channel proteins, catalogued into the Chloride channel accessory (CLCA) family. The first member of this family to be characterized was a respiratory epithelium, Ca2+-regulated, chloride channel protein isolated from bovine tracheal apical membranes. It was biochemically characterized as a 140 kDa complex. The bovine EClC protein has 903 amino acids and four putative transmembrane segments. The purified complex, when reconstituted in a planar lipid bilayer, behaved as an anion-selective channel. It was regulated by Ca2+ via a calmodulin kinase II-dependent mechanism. Distant homologues may be present in plants, ciliates and bacteria, "Synechocystis" and "Escherichia coli", so at least some domains within E-ClC family proteins have an ancient origin. The Chloride Intracellular Ion Channel (CLIC) Family (TC# 1.A.12) consists of six conserved proteins in humans (CLIC1, CLIC2, CLIC3, CLIC4, CLIC5, CLIC6). Members exist as both monomeric soluble proteins and integral membrane proteins where they function as chloride-selective ion channels. These proteins are thought to function in the regulation of the membrane potential and in transepithelial ion absorption and secretion in the kidney. They are a member of the glutathione S-transferase (GST) superfamily. They possess one or two putative transmembrane α-helical segments (TMSs). The bovine p64 protein is 437 amino acyl residues in length and has the two putative TMSs at positions 223-239 and 367-385. The N- and C-termini are cytoplasmic, and the large central luminal loop may be glycosylated. The human nuclear protein (CLIC1 or NCC27) is much smaller (241 residues) and has only one putative TMS at positions 30-36.