  Boolean networks are discrete dynamical systems for modeling regulation and
signaling in living cells. We investigate a particular class of Boolean
functions with inhibiting inputs exerting a veto (forced zero) on the output.
We give analytical expressions for the sensitivity of these functions and
provide evidence for their role in natural systems. In an intracellular signal
transduction network [Helikar et al., PNAS (2008)], the functions with veto are
over-represented by a factor exceeding the over-representation of threshold
functions and canalyzing functions in the same system. In Boolean networks for
control of the yeast cell cycle [Fangting Li et al., PNAS (2004), Davidich et
al., PLoS One (2009)], none or minimal changes to the wiring diagrams are
necessary to formulate their dynamics in terms of the veto functions introduced
here.
