The yeast two-hybrid experiment. The yeast two-hybrid system utilises the DNA binding domain and the activation domain of a yeast transcription factor. The bait proteins A and C are fused to a DNA binding domain, allowing them to bind to a binding site in the promoter region of a reporter gene. The prey protein B is fused to an activation domain that can activate the expression of a gene. (a) Protein B does not bind to protein A, therefore, the reporter gene is not activated. (b) Protein B binds to protein C, thereby activating the reporter gene.