of the 2619 structurally resolved protein-protein pairs recorded in the PDBbind database possess a K A-B stronger than 10 mM (Figure ). However, without inducing any de novo PPIs, a non-perturbative ligand obtains a cooperativity of 100 when binding at any surface of a weakly interacting protein pair, even if it does not stabilize the ternary complex at all (see Eq. ( ) and Figure ). One quickly notices that this "cooperative" ligand does not in fact introduce any cooperative phenomenon but merely reveals the intrinsic PPIs. This counter-intuititive findingcomes from the asymmetric nature of cooperativity (of the ligand) and becomes clear when the effective ternary dissociation constant in Eq. 1 is written with reduced cooperativity φ instead of cooperativity α,