Colocalization of a system of four polymers in Model A. (a) Probability of the different polymer equilibrium configurations as a function of the molecule binding affinity, E, at a fixed molecular concentration (c = 1.0%). Drastic changes in the probability of the different states are observed by increasing E. At low energies no colocalization is found, with the polymers floating independently. As E increases, a region exists where a single unstable couple is formed on average, but strong fluctuations are present. Finally, above a threshold value, a phase transition is crossed and the most likely state corresponds to the formation of two independent, stable couples. As illustrated, other states, having a smaller probability, also exist. (b) System-simplified phase diagram in the (c,E) plane. (Circles) Cross-over region (which disappears in the thermodynamic limit) where only one unstable couple is formed. Upper and the lower regions correspond, respectively, to the state where two couples and four independent polymers are the most probable configurations.