CLCN1 and Thomsen and Becker disease: However, apart from the function-degrading loss of CLC-1 channels, muscle fibers from human and animals with myotonia congenita have physiological, structural, and biochemical alterations, including hypertrophy (human, goats) or atrophy (mice) and alterations in fiber type composition (Beck et al., 1996; Jentsch et al., 2005).