Table 1. Determinants of cell-to-cell variability in gradual and ultrasensitive signal transduction.

Model		Dose-response behavior	Cell-to-cell variability	Note
Gradual	Eq. 1	gradual	high	variability most pronounced for weak stimulation
Feedback (upstream)	Eq. 8	very gradual	low	feedback primarily reduces pathway sensitivity (Km ,5) fluctuations
Feedback (downstream)	Eq. 12	very gradual	low	feedback primarily reduces maximal pathway activation (Xmax ,5) fluctuations
Distributed ultrasensitivity	Eq. 14	switch	low-high	variability can be regulated by phosphatase expression
Single-step ultrasensitivity	Eqs. 16, 17	switch	high	pathway threshold always highly variable
Single-step ultrasensitivity + basal transcriptional feedback	Eqs. 16–19	switch	low	switch-like decision making only for slow feedback possible
Single-step ultrasensitivity + coherent feedforward loop	Eqs. 16, 20	switch	medium	total variability is the combination of the individual branch variabilities