Polysaccharide storage myopathy (PSSM) is a hereditary muscle condition which occurs primarily in horses with Quarter Horse bloodlines such as Quarter Horses, Paint Horses, and Appaloosas. This muscle disease also occurs in other breeds such as Drafts, Draft crossbreeds, and Warmbloods. PSSM is a glycogen storage disorder and is characterized by the abnormal accumulation of the normal form of sugar stored in muscle (glycogen) as well as an abnormal form of sugar (polysaccharide) in muscle tissue. The unique feature of PSSM is that in horses with PSSM, sugar from the bloodstream is removed and transported into the muscle at a faster rate, making more glycogen than is seen in horses with normal copies of the gene. The primary clinical sign of this disease is muscle cramping or tying-up. However, clinical signs and severity may vary with different breeds. Signs are most commonly skin twitching, stiffness, firm painful muscles, sweating, weakness, and reluctance to move with light exercise. Horses with PSSM typically have calm dispositions and are in good body condition. In some horse’s symptoms may begin by 2 to 3 years of age while others can remain subclinical. PSSM cannot be cured but it can be managed using a proper diet and exercise routines. Only one copy of the mutated gene is needed to cause PSSM in horses. Unistel Medical Laboratories offers a test to determine whether horses have the mutated copies for the GYS1 gene which could result in Polysaccharide storage myopathy.