The GAL4 protein of Saccharomyces cerevisiae is one of the most thoroughly characterized transcriptional activators. As the N-terminal 147 amino acid residues of GAL4 are sufficient to mediate specific and strong binding to DNA, but are incapable of efficient transcriptional activation, this protein fragment has frequently been used to confer specific DNA binding in experiments examining transcriptional activation functions of heterologous proteins. This approach is facilitated by the finding that higher eukaryotes lack endogenous proteins that enhance transcription from the consensus GAL4-binding site. Fusions between GAL4 (amino acids 1-147) and activating domains from a variety of transcriptional regulatory proteins can activate transcription in yeast, plant, insects and mammalian cells. Fields and coworkers have taken advantage of these findings by the development of a unique “two-hybrid” system using GAL4 fusions in yeast to identify specific protein-protein interactions.