A key phenomenon in the study of cell-to-cell communication and
protein regulation is the all-or-none, ultrasensitive dose response, which
transforms a continuous input into a digital output. Multisite systems are
often used in conjunction with allosteric effects to create such a behavior.
In this talk I describe a non-allosteric mechanism for a multisite system to
present strongly ultrasensitive behavior. Applications are given to protein
activation through multisite phosphorylation, clusters of receptors and DNA
regulation through histone modifications.