  Recent experiments at the level of a single cell have shown that gene
expression occurs in abrupt stochastic bursts. Further, in an ensemble of
cells, the levels of proteins produced have a bimodal distribution. In a large
fraction of cells, the gene expression is either off or has a high value. We
propose a stochastic model of gene expression the essential features of which
are stochasticity and cooperative binding of RNA polymerase. The model can
reproduce the bimodal behaviour seen in experiments.
