  Systems biology approaches combining theoretical modeling with experiments
have been singularly successful in uncovering novel features of cellular
phenomena. One such feature is that of binary gene expression in which the
expression level is either low or high, i.e., digital in nature. This gives
rise to two distinct subpopulations in a population of genetically identical
cells. The fraction of cells in the high expression state is raised as the
strength of the inducing signal is increased indicating that the response is
not graded. In this review, we discuss the possible origins of binary gene
expression with emphasis on three principal mechanisms: purely stochastic,
positive feedback-based and emergent bistability. In the latter case, two
stable expression states are obtained due to an autoregulatory positive
feedback loop in protein synthesis along with cell growth retardation by the
proteins synthesized. The theoretical foundations of the observed phenomena are
described in each case.
