Figure 1.

The most general case of a constitutively expressing promoter. An inactive gene (black) is turned into an active gene (and vice versa). The active gene (blue and green) is transcribed into an mRNA (red), which is then translated to a protein (red ellipse). The mRNA and the protein are also degraded. Various rate constants $\mathcal{K}$ govern the time evolution of P(g,m,p|$\mathcal{K}$), the joint probability distribution of g (number of activated genes), results for m (number of mRNA molecules), and the parameterization of p (number of protein molecules).