The yeast two-hybrid system is a powerful technique for studying protein-protein interactions. Two proteins are separately fused to the independent DNA-binding and transcriptional activation domains of the Gal4p transcription factor. If the proteins interact, they reconstitute a functional Gal4p that activates expression of reporter gene(s). In this way, two individual proteins may be tested for their ability to interact, and a transcriptional readout can be measured to detect this interaction. Furthermore, novel interacting partners can be found by screening a single protein or domain against a library of other proteins using this system. It is this latter feature-the ability to search for interacting proteins without any prior knowledge of the identity of such proteins-that is the most powerful application of the two-hybrid technique.