a light on at night can be used to assist affected animals. The allele frequency of CSNB2 in Tennessee Walking Horses is reported to be about 10%, horses homozygous for CSNB2 are affected, while a significantly larger number of animals heterozygous for the condition are unaffected carriers. Genetic testing is available to aid in diagnosis, management, and breeding decisions. Type 1 Polysaccharide Storage Myopathy (PSSM1), a muscle disease which also affects several other breeds. It results in the abnormal accumulation of glycogen , leading to a breakdown of muscle fibers (Equine exertional rhabdomyolysis), causing muscle pain, weakness, skin twitching, sweating, and reluctance to move. Androgen Insensitivity Syndrome an X-linked recessive condition which affects male horses, where external genitalia fails to develop the animal is sterile. History The Tennessee Walker originated from the cross of Narragansett Pacer and Canadian Pacer horses brought from Kentucky to Tennessee starting in 1790, with gaited Spanish Mustangs imported from Texas. These horses were bred on the limestone pastures of Middle Tennessee, and became known as "Tennessee Pacers". Originally used as all-purpose horses on plantations and farms, they were used for riding, pulling and racing. They were known for their smooth gaits and sure-footedness on the rocky Tennessee terrain. Over the years, Morgan, Standardbred, Thoroughbred and American Saddlebred blood was also added to the breed. In 1886, Black Allan (later known as Allan F-1) was born. By the stallion Allendorf (from the Hambletonian family of Standardbreds) and out of a Morgan mare named Maggie Marshall, he became the foundation sire of the Tennessee Walking Horse breed. A failure as a trotting horse, due to his insistence on pacing, Black Allan was instead used for breeding. From his line, a foal named Roan Allen was born in 1904. Able to perform several ambling gaits, Roan Allen became a successful show horse, and in turn sired several famous Tennessee Walking Horses. The Tennessee Walking Horse Breeders' Association was formed in 1935. To reflect interest in showing horses, the name was changed in 1974 to the current Tennessee Walking Horse Breeders' and Exhibitors' Association (TWHBEA). The stud book was closed in 1947, meaning that since that date every Tennessee Walker must have both its dam and stud registered in order to be eligible for registration. In 1950, the United States Department of Agriculture recognized the Tennessee Walking Horse as a distinct breed. In 2000, the Tennessee Walking Horse was