In this form a fold change of mRNA expression can be correlated to the chance that a polymerase binds (Nns/P e(-(∆εpdS)/(KbT))). If the term describing RNA polymerase binding equals zero, the the chance that a RNA polymerase binds is equal to one and there is no change in gene expression. This will most likely be the case in a system without interference. Most biological systems, however, are dependent on regulators, repression is the main method of gene regulation in our designed circuit (see Kill-switch). The effect of repressors can be taken up in previous equation by adding the microstate ‘repressor protein bound’ as a regulatory region to the total statistical weight (see equation below). The term ‘repressor binding’ states that if a repressor is bound a polymerase cannot bind. The equation assumes a high basal level of expression in the absence of repressors. Note that like the previous equation, it takes the form of a Hill function, the gene expression fold change is dependent on the number of repressors