A long-ago ancestor of the modern domestic dog is alive today in the form of the canine transmissible venereal tumor (CTVT). This tumor was first identified in the late 1800s when it was found to be transferred to new hosts through tumor cells (1). We now know that the tumor is naturally transmitted between dogs by direct contact, primarily through coitus or activities that permit sloughing of cells (2). The tumors are rarely metastatic and most tumors regress within a few months, leaving previously infected dogs with immunity. DNA analysis provides strong evidence that all CTVTs studied thus far are from a single source, one that existed before the dispersal of dog breeds around the world (3, 4). On page 437 of this issue, Murchison et al. (5) describe the first whole-genome sequence of CTVT, sampled from two tumors: one from a random-bred Australian Aboriginal camp dog and the other from a purebred American cocker spaniel from Brazil.
