Method of screening for protein inhibitors and activators

The present invention relates to a method for determining whether a substance is an inhibitor or activator of a protein, the production of which by a cell causes a noticeable change in a different phenotypic characterization of the protein level in the cell per se, the method is characterized in that it comprises: a) providing a first cell line that produces the protein and exhibits the phenotypic response to the protein; b) providing a second cell line that produces the protein at a lower level than the first cell line, or does not produce any protein, and which presents the phenotypic response to the protein to a lesser degree or does not present it; c) incubating the substance with the first and second cell lines; and d) comparing the phenotypic response of the first cell line for the substance with the phenotypic response of the second cell line for the substance.