Fifty-two per cent of variants with dominant functional features were found in pedigrees with dominant inheritance of clinical symptoms (Fig. 4B), 14% in heterozygous probands with sporadic inheritance of myotonia congenita and a further 10% in dominant pedigrees although classified with ‘uncertain association with myotonia congenita’ as they were compound heterozygous with another CLCN1 variant and segregation data were not available to confirm which variant was associated with dominant inheritance or with a known pathogenic SCN4A variant. This evidence concerns the gene CLCN1 and Thomsen and Becker disease.