Transmissible venereal tumour (TVT) is biologically unique among all known cancers — it is a naturally transmissible cancer spread between dogs through direct contact, typically during mating. The tumour cells themselves are the infectious agent, transferred from one dog to another as living cells, much like a transplant. Remarkably, all TVT cells worldwide descend from a single dog that lived thousands of years ago, making this one of the oldest known cell lines. The tumour most commonly appears on the genital mucosa as a fleshy, cauliflower-like growth, though it can occasionally appear at other sites such as the nose or mouth. TVT is endemic in many tropical and subtropical countries with free-roaming dog populations but rare in countries with effective stray-dog control. The disease responds remarkably well to chemotherapy with vincristine, which is curative in the vast majority of cases.