In one aspect the disclosure relations to means and methods for identifying a protein or a DNA encoding the protein, involved in the production of a product by a micro-organism. In the methods the micro-organism is cultured under different culture conditions each of which exhibit a different level of the product that is produced by the micro-organism. The genetic expression of the genes of the micro-organism is compared with the level of the product, and groups of DNAs are identified that are involved in the production of the product by the micro-organism.