CLCN1 and myotonia congenita, autosomal recessive: Disease is due to a mutation of the skeletal muscle chloride voltage-gated channel one (CLCN1) gene located at chromosome 7q35 [3] and can be either dominant (Thomsen disease) or recessive (Becker disease). Mutations in the CLCN1 gene cause impaired muscle relaxation after a contraction leading to stiffness, cramping, and muscle rigidity.