Abstract: Provided is a screening method with which it is possible to determine the cause of a decrease or an increase in the amount of a target protein that is expressed by a cell. One or a plurality of embodiments include: conducting cultivation that includes bringing an assay cell into contact with a test substance, and measuring a relative amount (A) of the target protein in relation to an internal standard; conducting cultivation in which an assay cell is not brought into contact with a test substance, and measuring a relative amount (B) in relation to an internal standard; comparing the relative amount (A) and the relative amount (B); and, based on the comparison, selecting a candidate substance that induces instability and/or stability in the target protein.