SCN4A and Myotonia: In the case of the non-dystrophic myotonias, this has been extremely difficult mainly due to two factors: (1) the wide spectrum of symptoms and overlapping phenotypes present in both groups (Cl− and Na+ myotonic channelopathies) (24, 25, 47), and (2) both genes, CLCN1 and SCN4A, present high genotypic variability, with more than 200 or 65 different mutations already described, respectively (http://www.hgmd.cf.ac.uk/ac/index.php) (23, 43).