The electrical distance of the enhancement site. The EC₅₀ data from both roscovitine and nonroscovitine datasets (A) were normalized to the EC₅₀ value at 0 mV (for each dataset; B) and fit using the Woodhull equation (Eq. 1) to yield an electrical distance of ∼0.55. The data at V < 0 mV were not included in the fit.