Nico Katsanis, a Duke geneticist who hunts down the causes of rare illnesses, is one of a growing number of researchers choosing to work with zebrafish instead of rodents. Since scientists learned to selectively mutate zebrafish DNA in 1988—giving them the ability to turn the species into models of human diseases—the number of biomedical zebra-fish papers has skyrocketed, from 26 to 2,100 last year. The nonprofit Zebrafish International Resource Center, which sells 2,608 different genetically modified strains to researchers, lists 921 academic labs and companies that use the fish. "The field is on fire," says Leonard Zon of Harvard Medical School. Zon's lab, for example, has used fish models to study skin cancer, blood diseases, and stem cells. Others have created fish with DNA mutations linked to narcolepsy, muscle disorders, and the large head size associated with autism.