Every organism has proteins, which need to be expressed in order, in time, and in quantity. Viruses have several to hundreds of proteins. For a complex virus with hundreds of proteins, it may need to separate expression of early and late proteins and maintain precise ratios of different protein products expressed simultaneously. Because all virus proteins are expressed only once, we only need to consider the order of virus protein expressions, while the quantities of protein expressions are controlled by the feedback through the affinity of regulatory factors to the binding sites.