Principle of the Yeast Two-Hybrid System.

(A), (B) Two chimeras, one containing the DNA-binding domain (DB:blue circle) and one that contains a reporter activation domain (AD: half blue circle), are co-transfected into an appropriate host strain. (C) If the fusion partners (these proteins are yellow and red) interact, the DB and AD are brought into proximity and can activate transcription of reporter genes (here LacZ) suggesting a possible protein-protein interaction between the yellow and red proteins.