Abstract: The present invention relates to the recombinant modification of microorganisms, in particular actinomycetes. This is effected with the use of the recombination systems Dre-rox and Cre-lox. The present invention provides novel nucleotide sequences for the recombinases of the abovementioned recombination systems. Here, synthetic genes which have the codons preferred by actinomycetes and can therefore be used in these host organisms were produced. With the method according to the invention for modifying microorganisms, it is possible to produce overproduction strains which are of economic importance for the production of medical active substances.