Thomsen's and Becker's diseases are the most prevalent nondystrophic myotonias. 
Their frequency varies, according to different sources, from 1 : 100 000 to 1 : 
10 000. Thomsen's myotonia is autosomal dominant, and Becker's myotonia is 
autosomal recessive. Both diseases result from mutations of the CLCN1 gene 
encoding chloride ion channels of skeletal muscles. Molecular genetic analysis 
of the CLCN1 gene has been performed in patients with diagnoses of nondystrophic 
Thomsen's and Becker's myotonias living in the Russian Federation. A sample of 
79 unrelated probands with nondystrophic Thomsen's and Becker's myotonias and 44 
their relatives has been formed in the Laboratory of DNA Diagnosis of the 
Medical Genetic Research Center of the Russian Academy of Medical Sciences. 
Forty CLCN1 gene mutations have been found in a total of 118 chromosomes of 66 
probands, including 21 familial and 45 sporadic cases. About half the mutations 
detected (45%) have been found for the first time; they are not described in the 
SNP database (ncbi.nlm.nih.gov). The following mutations (substitutions) have 
been detected in more than one chromosome, accounting for a total of 59.3% of 
chromosomes with mutations: Glyl90Ser (5.9%), c.1437-1450del14 (9.3%), Ala493Glu 
(5.1%), Thr550Met (3.4%), Tyr686Stop (5.1%), and Arg894Stop (30.5%).