Dogs have a sexually transmitted cancer apparently spread via tumor cells that roam the world as parasites. Robin Weiss of University College London and his colleagues investigated canine transmissible venereal tumor, a malignancy found in the domestic dog and possibly in relatives such as the gray wolf and the coyote. Their DNA analysis of dog tissue samples from five continents reveals that the tumor cells did not belong to the dogs in which they festered. Instead they were all nearly genetically identical to one another and similar to cells from wolves or closely related dog breeds from China or Siberia. Judging by the number of mutations the cells accumulated, the cells' origin dates to sometime between 200 and 2,500 years ago, making them the oldest known cancer lineage. This cancer could yield clues on how tumors survive and elude the immune system, the investigators write in the August 11 Cell.