Phage display is a test to screen for protein interactions by integrating multiple genes from a gene bank into phages.

Like the two-hybrid system, phage display is used for the high-throughput screening of protein interactions. The principle of this method is summarized as follows:

1. The function of protein X is unknown. The protein is used to coat the surface of a small plastic dish.
2. All the other genes in the genome are expressed as fusions with the coat protein of a bacteriophage, so that they are displayed on the surface of the viral particle.
3. This phage-display library is added to the dish. After a while, the dish is washed.
4. Phage-displaying proteins that interact with protein X remain attached to the dish, while all others are washed away. DNA extracted from interacting phage contains the sequences of interacting proteins.

from: The Wellcome Trust