Cancer kills. Twenty years ago, the diagnosis of lymphosarcoma (a tumor of the lymph glands) in humans was almost invariably fatal. However, with the development of improved means to sub-classify this neoplasm and the tailoring of therapies that are subtype-specific, more and more forms of lymphosarcoma are treatable. One of the most important means of sub-classification of human tumors is based on the identification of chromosome abnormalities. In the dog, lymphosarcoma comprises one in five malignancies; however, the extent and identity of chromosome aberrations is still unknown. This is largely because the chromosomes of dogs were extremely difficult to identify with confidence. Recently, we have developed a set of canine chromosome-specific reagents that allow us to identify conclusively every dog chromosome. We propose to use these reagents to identify the chromosome aberrations associated with dog lymphosarcoma and to investigate the correlation between these aberrations and the clinical disease. Such an approach offers a means to potentially sub-divide this diverse disease in dogs, thereby offering new information of diagnosis, prognosis and therapy. Identification of specific chromosome aberrations will also help to investigate the correlation between the genetic etiologies in dogs with those in humans.