Polysaccharide Storage Myopathy (PSSM1) is a muscle disease that occurs primarily in horses with Quarter Horse bloodlines such as Quarter Horses, Paints, and Appaloosas, and draft breeds, although cases have been reported in more than 20 breeds. It is caused by a genetic mutation in the glycogen synthase 1 (GYS1) gene, which causes muscle cells to produce glycogen continually. This abnormal accumulation of sugar molecules in muscle cells leads to muscle cramping, soreness, and weakness. Horses with PSSM1 have a muscle disease characterized by the accumulation of abnormal complex sugars (glycogen) . There is no cure for PSSM, but it can often be managed successfully through diet and exercise. Feeding hay with less than 10 percent nonstructural carbohydrates (NSC) and providing regular exercise are good care practices for horses with PSSM. Approximately 50% of affected horses with PSSM1 show improvement under dietary management alone, and of those that adhere to dietary and exercise management, 90% have few to no episodes of tying-up.