A technique for detecting an interaction between two proteins by means of expression of a reporter gene in yeast cells. It can be used in high-throughput screening of libraries of genes or potential genes (open reading frames) from any species (e.g. mouse or human) to identify pairs of genes that encode interacting proteins. Essentially, a plasmid carrying a gene for the query protein, termed the ‘bait’, is introduced to numerous yeast cells, each of which contains a plasmid carrying a particular gene from the gene library, termed the ‘fish’. The plasmids are constructed such that both the bait and the fish proteins are expressed in the cell as hybrid proteins, in which the bait or fish protein is fused to a second protein involved in activating expression of a reporter gene. Binding of the bait and fish proteins inside the yeast nucleus allows activation of the reporter gene, and the yeast colony containing those particular genes can be pinpointed and analysed further.