Figure 6.

Analysis of one aspect of system response, the SLQB of cell-to-cell variation in system output. Shown is how we dissected cell-to-cell variation in reporter gene expression. Division of system into a pathway subsystem, P, that goes from receptor binding to transcription factor activation, and an expression subsystem, E, that goes from transcription factor activation to accumulation of reporter protein. By analyzing system output in combination with fluorescent reporter gene expression from constitutive promoters, we were able to find that the majority of cell-to-cell variation in total system output was due to cell-to-cell variation in a component of the expression subsystem, not in gene expression noise or in signal transmission variation [4].