Lens luxation is a debilitating inherited disease in which a defect of the lens suspensory apparatus leads to both lenses becoming loose within the eyes of adult dogs. This causes pain and damage to the eye, and commonly leads to glaucoma, threatening remaining vision. Treatment by surgical lens removal leads to major deficiencies in vision. The disease occurs in adult dogs of many terrier breeds, as well as Australian Cattle Dogs, Chinese Cresteds, Chinese Shar Pei, and Border Collies. We have used genomic mapping in two terrier breeds to define an interval in which the causative mutation must occur. There is insufficient variation in these breeds to allow fine mapping to identify the mutant gene. Under this grant we shall perform an analysis using Tibetan Terriers that suffer from the same condition. Preliminary data suggests that the mutation is located at the same place in this breed. The current study will allow us to confirm the mutation responsible for PLL in Tibetan Terriers is located in the same region of the genome as the equivalent mutation in the other two breeds and to greatly reduce the interval in which the mutation must occur. Candidate genes within the refined interval will be sequenced to define potential mutations.