Thomsen's (TM) and Becker's (BM) Myotonias are nondystrophic myotonias. At 
present, 150 mutations in the CLCN1 gene, which results in the development of TM 
and BM, have been described. c.2680C > T (p.Arg894*) is the most common 
mutation. In the Northern Scandinavian countries, the population frequency of 
this mutation is 0.87%, while in the Russian Federation, it is equal to 1.2% 
(this study). Based on the results of a molecular-genetic analysis of CLCN1 gene 
in patients with nondystrophic myotonias, the calculated frequency of TM and BM 
in Russia is 1:8165 and 1:710, respectively. We have conducted haplotype 
analysis using microsatellite markers and intragene SNP, which has shown that 
the prevalence of p.Arg894* mutation in Russia results from the founder effect, 
and the time of its scattering is 3680 ± 1240 years.