(A) Effect of the rad54∆ and rad52∆ mutations in spontaneous and MMS-induced uSCE. The frequency of spontaneous and MMS-induced recombinants was determined in mid-log phase cell cultures before (spontaneous uSCE) and after treatment with 0.02% MMS for 4 hours (DNA damage-induced uSCE). Thus, the frequency of DNA damage-induced uSCE includes the recombinants that arose spontaneously before MMS addition. The frequency of uSCE was determined at 0.02% MMS because at higher MMS concentrations the loss of viability of rad52∆ and rad54∆ mutants did not allow us to select for His+ recombinants. The mean and SEM of 5 independent fluctuation tests are shown; n.a., not applicable. Data information: 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. 