Effect of the lack of HR factors in spontaneous and MMS-induced mutagenesis. The frequency of spontaneous and MMS-induced forward mutagenesis at the CAN1 locus was determined in mid-log phase cell cultures before (spontaneous mutagenesis) and after treatment with 0.01% MMS for 4 hours (DNA damage-induced). Thus, the frequency of DNA damage-induced mutagenesis includes the mutants that arose spontaneously before MMS addition. The frequency of mutagenesis was determined at 0.01% MMS because at higher MMS concentrations the loss of viability of the rad mutants did not allow us to select for canavanine-resistant cells. Survival (%) after 4 hours in MMS relative to the wild type (taken as 100) is shown below the genotypes. The mean and SEM of 6 (wild type), 9 (rad54∆) and 3 (rest) independent fluctuation tests are shown. Asterisks indicate statistically significant differences according to an unpaired two-tailed Student's t-test (one, two and three asterisks represent p-values <0.05, <0.01 and <0.001, respectively). Only the statistical analysis of the effect of rad52∆, rad57∆ and rad51∆ on rad54∆-mediated mutagenesis is included for clarity. All mutants displayed statistically significant differences with the wild type and not with each other in the absence of MMS. 