Cancer is the leading cause of death in dogs. It is estimated that 1 in 3 domestic dogs will develop cancer, which is the same incidence of cancer among humans. Dogs can develop a variety of cancers and most are very similar to those found in humans. Dogs can develop carcinomas of epithelial cells and organs, sarcomas of connective tissues and bones, and lymphomas or leukemias of the circulatory system. Selective breeding of dogs has led certain pure-bred breeds to be at high-risk for specific kinds of cancer. Veterinary oncology is the medical study of cancer in animals, and can be diagnosed and treated by specialized veterinarians called veterinary oncologists. Causes Cancer is a complex, multifactorial disease. Carcinogenesis is linked with DNA mutations, chromosomal translocations, dysfunctional proteins, and aberrant cell cycle regulators. Cancer alters the DNA of cells and the mutated genetic material is passed on to daughter cells, resulting in neoplasms. The mutated DNA effects genes involved with the cell cycle, classified as either oncogenes or tumor suppressor genes. Oncogenes are responsible for cell proliferation and differentiation. Oncogenes responsible for cell growth are overexpressed in cancerous cells. Tumor suppressor genes prevent cells with erroneous cell cycles from replicating. Cancer cells ignore cell cycle regulators that control cell growth, division, and death. The histology of spontaneous tumorigenesis in canines is attributed to the multiplicity and complexity of the disease. The heterogeneity of its development encompasses inherited, epigenetic, and environmental factors. The selective breeding techniques used with domestic dogs causes certain breeds to be at high risk for specific cancers. Selection for specific phenotypes in dog breeding causes long-range linkage disequilibrium in their DNA. Certain areas of alleles have the tendency to separate less frequently than normal random segregation, which leads to long ranges of repeated DNA sequences. These repeated sequences caused by decreased genetic diversity within breeds, can lead to a high prevalence of certain diseases and especially cancer in breeds. It is believed that the breeding and inbreeding of domesticated canines for specific traits has significantly decreased nucleotide diversity in many pedigree dogs, making certain varieties of canines more susceptible to developing cancer. Symptoms Symptoms of cancer in dogs may include: Lumps (which are not always malignant, but should always be examined by a vet) Swelling Persistent sores Abnormal discharge from any part of the body Bad breath Listlessness/lethargy Rapid, often unexplained weight loss Sudden lameness Offensive odor Black,