During the cell cycle, replication errors can lead to spontaneous mutations, resulting in the appearance of genetic variants. Compared to their hosts, phages have much higher mutation and recombination rate, even those that use their host’s replication machinery. The estimated mutation rate of dsDNA such as lambda phage is ˜10-8 mutation/nucleotide/replication (m/n/r) i.e., ˜100 times higher than that of its host E. coli. The highly conserved DNA Mismatch Repair (MMR) system corrects >90% of replication errors, significantly contributing to the avoidance of mutations. In E. coli, the key MMR proteins are the MutSLH complex, UvrD helicase, and Dam methyltransferase, which methylates newly replicated GATC sites. MutSLH uses the latency required for this methylation to recognize and cut the newly synthesized strand, triggering the repair. It has been shown that in lambda phage GATC sites are under-methylated, probably because the level of intracellular methylase Dam becomes limiting. We investigated whether lambda's high mutation frequency is due to inefficient MMR as a result of under-methylation of GATC sites. To this end, we characterized the effect of modulating dam expression on lambda DNA methylation levels and mutant frequency. Our results show that increase in Dam level has a mutagenic effect in lambda.