İntroduction: Congenital Myotonia (CM) is a disorder related with the mutation in the skeletal muscle chloride channel gene (CLCN1). CM related to the mutation in the CLCN1 gene, is divided into two subgroups according to its inheritance patterns as autosomal dominant Thomsen disease and autosomal recessive Becker disease.

Clinical characteristics of two patients are presented here in this article, who were considered as Thomsen disease on the basis of history and physical examination findings and determined to carry a novel mutation in CLCN1 gene  never identified before, with a view to contribute to the genotype-phenotype correlation.

Cases: Two siblings, 11-year-old girl and the 3-year-old boy, applied with the complaints of difficulty in opening the hands after voluntary muscle contractions especially in the cold weather and discomfort in movements that lasted since they were one year old. DNA sequence analysis revealed that both siblings carried homozygously the c.1064+1G>A mutation within the 9th intron of the CLCN1 gene. This variance detected in the patients has not been identified in the literature before. Mutation Taster Bioinformatics program predicted that this variance could be the cause of a disease in line with clinical practice.

Discussion: The c1064+1G>A homozygous mutation (splice-site change mutation) within the 9. intron of the CLCN1 gene identified in our patients was not described before as a pathogenic mutation or polymorphism in the literature and international databases. Heterozygous CLCN1 gene mutations can cause a broad phenotypic spectrum.