A 7-month-old New Forest foal presented for episodes of recumbency and stiffness 
with myotonic discharges on electromyography. The observed phenotype resembled 
congenital myotonia caused by CLCN1 mutations in goats and humans. Mutation of 
the CLCN1 gene was considered as possible cause and mutation analysis was 
performed. The affected foal was homozygous for a missense mutation (c.1775A>C, 
p.D592A) located in a well conserved domain of the CLCN1 gene. The mutation 
showed a recessive mode of inheritance within the reported pony family. 
Therefore, this CLCN1 polymorphism is considered to be a possible cause of 
congenital myotonia.