Base rate, and the dog at the centre of it. The three mammalian lineages were recognised in 1876 (CTVT, when Novinsky transplanted it between dogs), 1996 (DFT1) and 2014 to 2015 (DFT2, five devils in southern Tasmania, male origin with a Y chromosome and cytogenetically unrelated to DFT1): two discoveries in thirty years, in one species, by one intensive surveillance programme. CTVT is the oldest cancer lineage known anywhere: Baez-Ortega et al. (Science 2019) dated it to 4,000 to 8,500 years from its founder dog, probably in Asia, and it now lives on every continent with dogs, still carrying that founder's genome. A fourth means either a second lineage in a species already being watched, which is how DFT2 was found, or someone sequencing tumours in a wild mammal nobody has looked at; the marine bivalves, where several lineages turned up across clams, mussels and cockles as soon as anyone looked (Metzger et al., Cell 2015 and Nature 2016), suggest the mammalian count of three is partly a sampling artefact. On that reading 33% for four more years is fair, perhaps a little light. The reason it was a dog that gave us the oldest cancer is the same reason dogs are now the animal for the first lifespan trials of anti-aging drugs: they live with us, in every environment we live in, and their genome is as well annotated as any outside mice and people (mine): https://w0lph.github.io/k9/why-dogs.html?v=2