The present invention provides a method for identifying or selecting from a population of eukaryotic cells cultivated on or in a medium containing at least one compound, cells which have a metabolic advantage as a result of having being transformed, wherein: (i) the cells are transformed with a nucleotide sequence or a co-introduced nucleotide sequence one of which comprises a region which: (a) encodes a protein which is involved in the metabolism of the compound, and/or (b) regulates the activity of the protein and (ii) the compound is mannose or xylose or a derivative or a precursor of these, or a substrate of the protein, or is capable of being metabolized by the transformed cells into such a substrate, with the proviso that the compound is not mannos when the proteine is mannose 6 phosphate isomerase. The invention also includes a method according to the preceding paragraph wherein the compounds are are not so limited with the proviso that an agent which reduces the toxicity to the cells of the compound is added to the medium. It is preferred that where a toxicity-reducing agent has been added to the culture medium, the compound is mannose and the nucleotide or co-introduced nucleotide sequence encodes mannose-6-phosphate isomerase.Procédé didentification ou de sélection, à partir dune population de cellules eucaryotes cultivées sur ou dans un milieu renfermant au moins un composé, de cellules présentant un avantage métabolique dû à leur transformation. Selon le procédé, (i) on transforme les cellules à laide dune séquence nucléotidique ou à laide dune séquence nucléotidique co-introduite dont lune comporte une région pouvant (a) coder une protéine intervenant dans le métabolisme du composé, et/ou (b) réguler lactivité de la protéine et (ii) le composé est la mannose, la xylose ou leur dérivé ou précurseur, ou bien un substrat de la protéine, ou bien il peut être métabolisé en un substrat de ce type par les cellules transformées, à condition que le composé ne pu
