Abstract: A process is disclosed for preparing a protein by a eukaryote transformed by multicopy integration of an expression vector into the genome of a yeast, such as Saccharomyces, Hansenula, and Kluyveromyces, or of a mould such as Aspergillus, Rbizopus and Trichoderma, the expression vector containing both an "expressible gene" encoding the protein and a so-called "deficient selection marker meeded for the growth of the yeast or mould in a specific medium", such as the LEU2d, TRP1d or URA3d gene, in combination witha ribosomal DNA sequence, resulting in stable high copy integration of 100-300 copies per cell. This multicopy integration results in an increased production of the desired protein, which can be guar .alpha.-galactosidase, an oxidase or a hydrolytic enzyme such as a lipase.