Tumors of the thyroid gland account for approximately 1 to 4 percent of all tumors in dogs. Of these, 70-100 percent are malignant, and tumors formed from follicular cells are the most common cell type diagnosed. In addition, approximately 33 percent of dogs with thyroid tumors will have spread of the cancer by the time they are diagnosed, and up to 90 percent of dogs will experience spread of the cancer through the course of the disease if left untreated. Prognosis is usually determined by the size of the tumor and extent of local tissue invasion at the time of definitive treatment. Thyroid tumors invade surrounding tissues (larynx, trachea, esophagus, and major vessels of the neck) and are considered highly vascular. As the tumors grow, they can cause breathing and eating difficulties, which may necessitate euthanasia. Our hypothesis for this study is that there are specific differences in the expression patterns of genes that are associated with a tumors ability to be invasive and that these differing expression patterns can be capitalized upon to allow the development of disease modifying therapies. We therefore propose to compare invasive and non-invasive thyroid tumors using microarray analysis.