al biotechnology ny 1992 august 10 8 894 8 ueda et al biochim biophys acta 2003 mar 17 1631 2 160 8 ko et al appl environ microbiol 2006 june 72 6 4207 13 hara et al arch microbiol 2001 november 176 5 364 9 kanayama et al j bacteriol 1998 february 180 3 690 8 cells preferred according to the invention are characterized in that the decrease in enzymatic activity is achieved by modification of at least one gene comprising one of the sequences selected from the previously stated nucleic acid sequence groups a and b the modification being selected from the group comprising preferably consisting of insertion of foreign dna into the gene deletion at least of parts of the gene point mutations in the gene sequence and subjecting the gene to the influence of rna interference or exchange of parts of the gene with foreign dna in particular of the promoter region foreign dna means in this context any dna sequence that is u201c foreign u201d to the gene and not to the organism i e even candida tropicalis endogenous dna sequences can in this context function as u201c foreign dna u201d in this context it is in particular preferable for the gene to be interrupted by insertion of a selection marker gene therefore the foreign dna is a selection marker gene the insertion preferably having been effected by homologous recombination into the gene locus in this context it may be advantageous if the selection marker gene is expanded with further functionalities which in their turn make subsequent removal from the gene possible this can be achieved for example with a cre loxp system with flippase recognition targets frt.