Fusion of proteins to DNA-binding domains or transactivation domains has become an extremely powerful approach to study the transcriptional properties of proteins and to characterize and clone interacting proteins (1,2). The yeast transcriptional activator protein GAL4 contains the sequences necessary for DNA-binding, dimerization and nuclear localization within its N-terminal 147 amino acids (3). A dimer of this fragment can bind specifically to a 17-bp DNA recognition sequence, which displays an imperfect dyad symmetry (4). It seems that higher eukaryotic cells do not possess proteins with GAL4-specific binding activity. Recombinant proteins between GAL4 and fused protein sequences of interest can be used to investigate two types of questions. First, potential protein/protein interactions can be detected by using an electrophoretic mobility shift assay (EMSA). This approach allows the discrimination of radioactively labeled unbound DNA, complexes between the GAL4 fusion protein and its cognate DNA, and more slowly migrating complexes containing additional protein(s) binding to the protein sequences fused to the GAL4 protein. Second, the potential transcriptional activity of regulatory proteins can be assayed by fusing them to the DNA-binding domain of GAL4 and testing their transcriptional activity on well-characterized promoters bearing Gal4 binding sites in a cell-free assay (for review, see ref. 1). This strategy has the advantage that transcription extracts have not to be depleted of the transcription factor of interest. It also allows the direct comparison of transactivation domains from different transcription factors regardless of the effects probably resulting from then DNA-binding domains.