Figure 1.

Interpreting results on direct physical interactions from Y2H experiments. (a) The observation of interactions A-B and B-C in a Y2H experiment does not indicate whether the two interactions can take place simultaneously (center) or whether they are exclusive of each other (right). (b) The ability of two proteins to interact may depend on post-translational modifications whose presence or absence may be actively regulated. Proteins D and E interact (center) in the absence of a certain post-translational modification (red shape), whose presence inhibits the interaction (right).