From a genetic perspective, this de novo heterozygous SCN4A allele arose independently in two families with the probands having severe myotonic syndromes with overlapping features, and the variant was not in unaffected family members or in public databases (gnomAD_v2.1.1 or ExAC, although the Moroccan population may be under represented). This evidence concerns the gene SCN4A and myotonic syndrome.