the homozygous form (H/H) of the gene, and though heterozygous (H/N) horses are still eligible for registration, altering that status is currently being discussed. Additionally all Quarter Horses born 2007 or later that are confirmed to be descendants of Impressive must carry a note about the risks of HYPP on their registration papers. Due to HYPP, the halter classes are undergoing significant changes. Halter classes are dominated by the Impressive bloodline. Impressive, a very prolific halter horse, brought to the stock breeds the muscle mass that became popular in halter competition. This muscle mass is linked to HYPP, and as the condition is reduced within the breed, the style of horse in halter classes is also likely to change. Already there have been rule changes, including the creation of a "Performance Halter class" in which a horse must possess a Register of Merit in performance or racing before it can compete. Malignant hyperthermia. A causative mutated allele, ryanodine receptor 1 gene (RyR1) at nucleotide C7360G, generating a R2454G amino acid substitution. has been identified in the American Quarter Horse and breeds with Quarter Horse ancestry, inherited as an autosomal dominant It can be caused by overwork, anesthesia, or stress. Hereditary Equine Regional Dermal Asthenia (HERDA), also known as hyperelastosis cutis (HC). This is caused by an autosomal recessive gene, and thus, unlike HYPP, HERDA can only be transmitted if both parents carry the gene. When a horse has this disease, there is a collagen defect that results in the layers of skin not being held firmly together. Thus, when the horse is ridden under saddle or suffers trauma to the skin, the outer layer often splits or separates from the deeper layer, or it can tear off completely. It rarely heals without disfiguring scars. Sunburn can also be a concern. In dramatic cases, the skin can split along the back and even roll down the sides, with the horse literally being skinned alive. Most horses with HERDA are euthanized for humane reasons between the age of two and four years. The very hotly debated and controversial theory, put forth by researchers at Cornell University and Mississippi State University is that the sire line of the great foundation stallion Poco Bueno is implicated as the origin of the disease. As of May 9, 2007, Researchers working independently at Cornell University and at the University of California, Davis announced that a DNA