Seymour Benzer and Ernst Freese compared spontaneous and 5-bromouracil-induced mutants in the rII gene of the bacteriophage T4; the mutagen increased the mutation rate (rII⁺ → rII) several hundred times above the spontaneous mutation rate. Almost all (98 percent) of the 5-bromouracil-induced mutants could be induced to revert to wild-type (rII → rII⁺) by 5-bromouracil treatment, but only 14 percent of the spontaneous mutants could be induced to revert to wild-type by this treatment. Discuss the reason for this result.