Escherichia coli MutS, MutL and MutH proteins act sequentially in the mismatch repair system (MMRS), which repairs DNA replication errors. Upon binding to the MutSL-DNA complex, the endonucleotyc activity of MutH is activated, and cleaves DNA 5` to the Dam hemimethylated -GATC- sequence in the unmethylated strand, directing repair to this strand. It has been reported using different experimental systems that overexpression of MutS or MutL is able to reduce the mutation rate in different organisms. However, the mechanism/s responsible for this effect is not known. In one of these systems it has been reported that this effect is not the result of an effect on the GO repair pathway. Here we show that a similar effect in the reduction of the mutation rate is also visible in a Dam mutant strain of E. coli. As in this strain the MMRS is non functional, our result indicates that the MMRS is probably not involved in the reduction of the mutation rate  effect after overexpression of MutS or MutL. Experiments in order to determine the mechanism responsible for this effect are under development.