Fibrosarcoma is an aggressive cancer of cats that is difficult to treat effectively. Some of the cancer cells found within these tumors produce a protein called KIT. Studies involving several types of cancer in people and dogs have shown mutations in this protein. These mutations can lead to cancer formation and increased tumor growth. Successful treatment of these cancers in people and dogs has been achieved through chemotherapy directed at the KIT protein. The purpose of this study is to determine whether these mutations are also present in the cancer cells of cats with fibrosarcoma. If mutations are found, it is possible that use of the same chemotherapy may improve the treatment outcome in cats with fibrosarcomas.