Sometimes it starts in the testes, but in extremely rare cases they believe that it is of primordial origin. For example, when you are developing as an embryo,

Sometimes it starts in the testes, but in extremely rare cases they believe that it is of primordial origin. For example, when you are developing as an embryo, perhaps the germ cell gets stuck in the chest and takes many years to mutate. But they don’t really know how this works. Very weird stuff.