Spontaneous mutation rates (with 95% confidence intervals) to (A) 5-FOA resistance and (B) canavanine resistance. Differences in mutation rates between the pol1-L868M exo1Δ and other two double mutant strains are statistically significant (p <0.05), as indicated by asterisks. Deleting EXO1 results in a 46-fold and a 12-fold increase in mutation rates to 5-FOA resistance compared to the corresponding single pol mutants; pol1-L868M and pol3-L612M, respectively. For canavanine resistance, deletion of EXO1 in a pol mutator background causes a mutation rate increase of 52-fold and 14-fold, compared to pol1-L868M and pol3-L612M, respectively.