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→ Disorder of the sugar metabolism of the muscles causing muscle damage.


  • Reluctance to move
  • Atactic gait/coordination problems
  • Muscle tremors
  • Frequent tying up/tension
  • Excessive sweating
  • Gait changes (cross firing or disunited canter/bunny hopping/rope walking)

General Information:

  • Degenerative muscle disease that manifests itself in a disorder of the sugar metabolism of the muscles.
  • Symptoms usually occur during/shortly after physical exercise.
  • Excessive activity of the enzyme glycogen synthetase 1 → Abnormal accumulation of glycogen granules in muscle cells.

Inheritance: incomplete autosomal dominant/semidominant
→ Horses with one or two copies of the variant (n/P1 or P1/P1) are affected. Incomplete or semidominant means that animals with one copy (n/P1) may show milder symptoms and a later age of onset than animals with two copies (P1/P1).

Possible genotypes:

Genotype: The horse is: Effects:
n/n normal. The horse does not have any copy of P1 and therefore cannot pass it on to any offspring.
n/P1 affected (heterozygous). The horse has one copy of P1 and will pass it on to approximately 50% of its offspring. These 50% are at risk of developing PSSM1.
P1/P1 affected (homozygous). The horse has two copies of P1 and will pass it on to 100% of its offspring. All offspring will be at risk of developing PSSM1.


  • Affected horses (n/P1 or P1/P1) should only be bred after careful consideration and with the advice of a genetic expert and veterinarian. Please contact us for consultation and support.

Test information: This test identifies a mutation in the GYS1 gene.
This test is performed by partner laboratory.

McCue, ME., Valberg, SJ., Lucio, M., Mickelson, JR. :
Glycogen synthase 1 (GYS1) mutation in diverse breeds with polysaccharide storage myopathy. J Vet Intern Med 22:1228-33, 2008. Pubmed reference:18691366. DOI:10.1111/j.1939-1676.2008.0167.x

McCoy, A.M., Schaefer, R., Petersen, J.L., Morrell, P.L., Slamka, M.A., Mickelson, J.R., Valberg, S.J., McCue, M.E. :
Evidence of Positive Selection for a Glycogen Synthase (GYS1) Mutation in Domestic Horse Populations. J Hered :, 2013. Pubmed reference: 24215078. DOI: 10.1093/jhered/est075.

McCue, ME., Valberg, SJ., Miller, MB., Wade, C., Dimauro, S., Akman, HO., Mickelson, JR.: Glycogen synthase (GYS1) mutation causes a novel skeletal muscle glycogenosis. Genomics 91:458-66, 2008. Pubmed reference: 18358695. DOI:10.1016/j.ygeno.2008.01.011.

Further information is available at Online Mendelian Inheritance in Animals.

Breeds affected:

Most breeds

Test #: H128

Samples: Blood, Hair

Price: 39 € (net)

Time required: 10-14 days

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