Fig. 3

Optimization of the pre-incubation time. The pre-incubation time was set at 0 min, 20 min, 1 h, or 2 h. The protein content in the S9 Mix was set at 1.2 mg/plate. The plots shown indicate the ratio of the average number of revertant colonies observed (treated: solvent control) in two plates and the number of spontaneous revertants. The number of spontaneous colonies per plate ranged from 100 to 133, varying slightly in response to the different pre-incubation times used