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 or by homologous recombination cells preferred according to the invention are characterized in that they are blocked in their u03b2 oxidation at least partially preferably completely as this prevents outflow of substrate and therefore higher titres become possible examples of candida tropicalis cells partially blocked in their u03b2 oxidation are described in ep0499622 as strains h41 h41b h51 h45 h43 h53 h534 h534b and h435 from which a candida tropicalis cell preferred according to the invention is derived other candida tropicalis cells blocked for u03b2 oxidation are described for example in wo03 100013 in this context cells are preferred for which the u03b2 oxidation is caused by an induced malfunction of at least one of the genes pox2 pox4 or pox5 therefore in this context cells are preferred that are characterized in that a candida tropicalis cell preferred according to the invention is.