The nucleotide change c.1453A>G (rs146457619; gnomAD 0.04%; ClinVar 280101) in exon 13 of CLCN1 yields the missense p.Met485Val which has been reported in a number of studies with myotonia congenita, both in homozygous and compound heterozygous state, while it was detected in heterozygote state in unaffected individuals (6–11). This evidence concerns the gene CLCN1 and Thomsen and Becker disease.