In 1973, Bruce N. Ames, a professor of biochemistry at the University of California, Berkeley, introduced a new assay for use in evaluating carcinogenicity. The test relied on four mutant strains ofSalmonellathat Ames’s group had developed, drawing on years of experience using such bacteria in studies of metabolism and mutagenesis. These strains were deficient in their ability to synthesize a particular amino acid, histidine, so they required this supplement in the growth media. Each of the four strains could be used to genetically screen compounds inducing a specific kind of mutation in the DNA sequence. These registered as