The basic function of restriction endonucleases and methyltransferases is protection of the host genome against foreign DNA. Their recombination and transposition related functions are still being discussed. Some authors postulate that R-M genes may act as selfish genetic elements. Restriction endonucleases are indispensable tools in molecular biology. As these enzymes recognize DNA sequence very specifically, they serve as a model for protein-DNA interaction. Restriction-modification genes have also played the same role as a model for evolutionary studies as well as protein structure - function relations. So far, there have been more than 3500 bacterial strains studied which possessed R-M genes of more than 280 different specificities towards recognition sequence and cleavage sites. They became a very good commercial product for many biotechnological companies. At present, in a genome sequencing era, R-M genes seem to be much more common than it was thought before.