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al o f P hy si ol og y Vertebrate skeletal muscle is distinguished by its large resting chloride conductance (gCl), a property that is maintained by intact, voltage-gated skeletal muscle chloride channels, ClC-1 (Steinmeyer et al. 1991). Mutations in CLCN1, the gene encoding the ClC-1, lead to autosomal dominant or recessive myotonia congenita in humans (Koch et al. 1992; George et al. 1993). This disorder o