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Polysaccharide Storage Myopathy Type 1 (PSSM1)


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

Instructions for ordering:

  1. Enter the information of the first horse for which you want to order the test
  2. Add to cart
  3. Go to cart to check out, or
  4. Enter the information for a second horse
  5. For sample collection follow the instructions here.
  6. You can use this template to send in hair samples.
  7. Please also send the completed order form together with the samples.

For a breeder or veterinarian discount please contact us before ordering.

Any information you include here is optional, but will be included on the certificate of results (except “Aditional Info”).

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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.


  • Reluctant 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

Inheritance and Genotypes

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 advise of a genetic expert and veterinarian. Please contact us for consultation and support.


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.

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