Simulated stochastic gene expression of Bcl-6, Blimp-1, and Pax5 in the B cell state. The histograms (right panels) generated from a population of 105 simulated cells illustrate the degree of variability in protein abundance (noise). The noise level is quantified by the coefficient of variation, σ/μ (where σ is the standard deviation and μ the mean). Due to the low abundance in B cells, Blimp-1 expression is much more noisy (larger σ/μ) than Bcl-6 and Pax5, and fluctuates in a pulsatile manner. The stochastic, pulsatile expression of Blimp-1 is primarily responsible for the heterogeneous switching response of individual B cells to LPS stimulation.