Gene activation in early development is highly dependent on precise concentrations of trans-acting factors for the activation of different genes at differing points in the embryo. Thus, not only is the presence or absence of a particular trans-activator or repressor relevant in determining gene activation, but also the concentration of the regulatory protein must be above or below a certain threshold for proper gene regulation. Signaling pathways in somatic cells are thought to represent cascades of on/off switches, mediated most commonly by phosphorylation. Here we demonstrate a quantitative mechanism for regulating the level of a component of the IFN-gamma signaling pathway that in effect represents the differential sensitivities of STAT1, IFN-regulatory factor-1, and class II trans-activator (CIITA) to IFN-gamma. Unlike developmental gene regulation, in which specificity of gene activation is a function of regulatory protein concentrations, specificity of gene activation in the IFN-gamma signaling pathway is regulated by the duration of the activation of the primary IFN-gamma-regulatory protein, STAT1. This result most likely explains previously reported data indicating that a minimum amount of IFN-gamma is required for MHC class II gene activation despite the fact that the level of the IFN-gamma-inducible factor directly required for MHC class II induction, CIITA, directly correlates with the level of MHC class II expression. The induction of a high level of CIITA is dependent on sustained IFN-gamma signaling. The possible implications of this result for tumorigenesis are discussed.