Since the invention of the cDNA array technology in the mid-1990s,many workers in the field have assumed that cDNA arrays enabled them to infer transcription rates (TR) from transcript levels (TL) measured with cDNA arrays.Based on this assumption,attempts have been made to identify the individual genes responsible for TL changes induced by specific perturbations of cells.But recent measurements of both TR and TL in budding yeast [1] and human lung cells [2] indicate that TR is related to TL in a complex manner,as evidenced by the fact that,when fold changes in TR are plotted against those of TL,highly nonlinear trajectories were found.