Cell-death noise. The cell-death noise, defined as the standard deviation over the mean of the ATP threshold distribution, increases with the expression of Bcl-xL. Its inferred value for the cell types "Bcl-xL", "Bcl-xL 1E1", and "Bcl-xL 4.1" is 0.28, 0.56, and 0.88, respectively. The mean (fmol/cell), variance (fmol2/cell2), and noise (dimensionless) are shown as a function of Bcl-xL expression normalized by that of the cell type "Bcl-xL". Bcl-xL expression reported in Western blots of Ref.  was quantified with the program ImageJ http://rsbweb.nih.gov/ij/.