The present invention provides a method for identifying or selecting from apopulation of eukaryotic cells cultivated on or in a mediumcontaining at least one compound, cells which have a metabolic advantage as aresult of having being transformed, wherein: (i) the cellsare transformed with a nucleotide sequence or a co-introduced nucleotidesequence one of which comprises a region which: (a) encodesa protein which is involved in the metabolism of the compound, and/or (b)regulates the activity of the protein； and (ii) the compound ismannose or xylose or a derivative or a precursor of these, or a substrate ofthe protein, or is capable of being metabolized by the transformedcells into such a substrate, with the proviso that the compound is not mannoswhen the proteine is mannose 6 phosphate isomerase. Theinvention also includes a method according to the preceding paragraph whereinthe compounds are are not so limited with the proviso thatan agent which reduces the toxicity to the cells of the compound is added tothe medium. It is preferred that where a toxicity-reducingagent has been added to the culture medium, the compound is mannose and thenucleotide or co-introduced nucleotide sequence encodesmannose-6-phosphate isomerase.
