Figure 5. The probability distribution function and the quasi-potential of the genetic toggle switch without a load.

(A). The probability distribution function of a toggle switch without a load. The x- and y- axes here represent the number of molecules of Repressor 1 and Repressor 2 respectively, while the z-axis is the frequency of its occurrence. Note that the distribution is symmetric as expected. (B). The quasi-potential of the symmetric toggle switch, showing the symmetric double-well potential constructed by taking the negative logarithm of the probabilities shown in (A). A small offset of 0.001 was added to the probabilities to prevent taking the logarithm of zero. This does not change the shape of the well.