  The copy number of any protein fluctuates among cells in a population;
characterizing and understanding these fluctuations is a fundamental problem in
biophysics. We show here that protein distributions measured under a broad
range of biological realizations collapse to a single non-Gaussian curve under
scaling by the first two moments. Moreover in all experiments the variance is
found to depend quadratically on the mean, showing that a single degree of
freedom determines the entire distribution. Our results imply that protein
fluctuations do not reflect any specific molecular or cellular mechanism, and
suggest that some buffering process masks these details and induces
universality.
