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Paramyotonia congenita (OMIM 168300) is a non-dystrophic myopathy caused by mutations in the SCN4A gene that sometimes can be confused with myotonia congenita. Another disease also caused by mutations in the gene SCN4A is called myotonia aggravated by potassium (OMIM 170500, 613345). It is estimated that more than 20% of patients with suspected myotonia congenita suffer paramyotonia congenita. The two related SCN4A phenotypes exhibit an autosomal dominant inheritance and are the result of mutations that cause an increase in the function of the protein coded by this gene. In this study we present a case of paramyotonia congenita in a family with several affected members and in which a mutation in the SCN4A gene was identified. Evolutionary conservation data and predictive algorithms of pathogenicity allow us to conclude that this DNA variant is the cause of the disease in this family., (Copyright © 2017 Elsevier B.V. All rights reserved.)