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Equine Myotonia

Myotonia (MYT)



Gene or region:  CLCN1

Reference allele: A

Mutant allele: C

Affected Breeds: New Forest Pony


Research Confidence:                            Moderate confidence, mutation was not found in 56 control horse of 13 different breeds, and mutations in the same gene result in myotonia in other species


What it does:  


Myotonia is a rare disorder involving a slowed relaxation of muscles after contraction. The most well-known example of myotonia is “fainting goats,” a breed that is characterized by sudden rigidy and/or falling over when startled. In the single documented horse, this resulted in a protruding third eyelid when excited, as well as problems with muscle stiffness (and thus difficulty in rising and movement). This mutation is believed to be a more recent occurrence within the New Forest Pony breed.


CLCN1 is a voltage-dependent chloride channel involved in signaling in muscles. Mutations in this gene have been associated with myotonia is humans, dogs, and goats. This specific mutation described in the horse results in an amino acid change, which likely alters the function of the encoded protein.





Wijnberg ID et al., “A missense mutation in the skeletal muscle chloride channel 1 (CLCN1) as candidate causal mutation for congenital myotonia in a New Forest pony.” (2012) Neuromuscul Disord. 22: 361-7. PMID: 22197188

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