The mice scurrying around their cage in Katsuhiko Hayashi's laboratory do not look remarkable. They run, eat and sleep like others of their kind. But these eight rodents have an unusual origin story, one that Hayashi, a reproductive biologist at Kyushu University in Japan, revealed two years ago in the pages of Nature. The tawny-colored mice, he and his colleagues announced, did not spring from the mating of sperm and egg. On their mother's side, their roots trace to a reprogrammed skin cell.