Ames could now test thousands of chemicals to create a catalog of chemicals that increased the mutation rate—mutagens. And as he populated his catalog, he made a seminal observation: chemicals that scored as mutagens in his test tended to be carcinogens as well. Dye derivatives, known to be potent human carcinogens, scored floridly, causing hundreds of colonies of bacteria. So did X-rays, benzene compounds, and nitrosoguanidine derivatives—all known to cause cancers in rats and mice. In the tradition of all good tests, Ames’s test transformed the unobservable and immeasurable into the
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