Abstract: 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; andii) 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 mannose when the protein is mannose 6 phosphate isomerase.