Abstract: An expression system for producing a protein in a filamentous fungus, consisting of a) a host organism selected from the species Trametes and Polyporus and b) a DNS vector containing a selection marker gene. This selection marker gene code is a protein which allows the selection of positive transformants after the transformation of the host organism and which is selected from the group of: antibiotics-resistant genes coding for proteins which eliminate the growth-inhibitory effect of antibiotics against which the host organism is not resistant. Genes coding proteins which are capable of a color-causing reaction; and genes complementing a genetic defect of the host organism (auxotrophy), the expression of the selection marker gene is controlled by at least one active genetic regulation element in the host organism, and c) a DNS vector containing a gene which codes the protein to be produced.