CLCN1 and Thomsen and Becker disease: Two forms of myotonia congenita have been described, the autosomal dominant form, better known as Thomsen disease (MIM #160800) [3] due to heterozygous pathogenic variants in CLCN1, and the autosomal recessive form, known as Becker disease (MIM #255700) due to bi-allelic pathogenic variants in CLCN1 gene.