An assay for detecting in vivo protein-protein interactions, which depends upon expression of a reporter gene, usually in yeast. As transcription of eukaryotic enzymes are regulated by transcriptional activators having distinct DNA-binding and transcriptional activator domains, the coding sequences for each of these domains can be fused with coding sequences for each of the two proteins to be tested; if the proteins bind, the DNA-binding and transcriptional activator domains will be reunited, permitting transcription of the reporter gene. Typically, the system is used to detect a binding partner of a known protein (encoded in the bait vector as a chimaera with the DNA binding domain of the transcription factor) to a candidate partner (encoded in the prey vector as a chimaera with the activation domain of the transcription factor), which is the homologue of a known binding partner or is encoded by a cDNA library.