The GYS1 gene mutation that is causative of Type 1 Polysaccharide Storage Myopathy (PSSM) has been identified in more than 20 breeds of horses. However, the GYS1 mutation frequency or Type 1 PSSM prevalence within any given breed is unknown. The purpose of this study was to determine the frequency of the GYS1 mutation and prevalence of genetic susceptibility to Type 1 PSSM in selected breeds from Europe and North America. The GYS1 mutation was detected in 11 breeds, including, in order of increasing allele frequency, Shires, Morgans, Appaloosas, Quarter Horses, Paints, Exmoor Ponies, Saxon-Thuringian Coldbloods, South German Coldbloods, Belgians, Rhenish German Coldbloods and Percherons. The prevalence of genetic susceptibility to Type 1 PSSM in these breeds varied from 0.5% to 62.4%. The GYS1 mutation was not found in the sampled Thoroughbreds, Akhal-Tekes, Connemaras, Clydesdales, Norwegian Fjords, Welsh Ponies, Icelandics, Schleswig Coldbloods or Hanoverians, but failure to detect the mutation does not guarantee its absence. This knowledge will help breed associations determine whether they should screen for the GYS1 mutation and will alert veterinarians to a possible differential diagnosis for muscle pain, rhabdomyolysis or gait abnormalities.