## Receptor Occupancy at Equilibrium.

**(A)** Simulation results for the number of active receptors as a function of time. Each color represents a different realization of the process. The thick, solid, black line is the mean from the data (5035 Ste2*). The thin, solid, black lines represent one standard deviation away from the mean, as calculated from the data (±52 Ste2*). The thick, dashed, black line is the theoretical mean calculated from Eq 1a (5027 Ste2*). The thin, dashed, black lines are one theoretical standard deviation from the mean as calculated from Eq 1b (±50 Ste2*). **(B)** A histogram of the data in **(A)**. The vertical lines are equivalent to those in **(A)**. The red curve shows the theoretical distribution. **(C)** A plot of time-averaged receptor occupancy. Each time point displays the average occupancy of the preceding 10 minutes. No average is available for t < 10min. The black lines are similar to those in **(A)**. The simulation mean is 5034 ± 23 Ste2*, and the theoretical mean is 5027 ± 19 Ste2*. The theoretical mean is again calculated with Eq 1a, but the time-averaged standard deviation is calculated from Eq 5 and the relation described in the text. **(D)** A histogram of the data in **(C)**. The time-averaged distribution is much narrower (σ = 23 Ste2*) than the instantaneous distribution (σ = 52 Ste2*). The parameters used for these simulations are reported in **Table 1**, with binding and unbinding rates of 1.6×10^{6} (M·s)^{-1} and 0.011 s^{-1}, respectively, and no ligand gradient.