Protein-protein interaction is the basis of many phenomena including replication,transcription, secretion, signal transduction,metabolism et al. Designed genetically,twohybrid system can be used to detect protein-protein interaction in vivo. An unknown protein which interacts with certain target protein can also be found by using this method.Consequently, the DNA sequence encoding the unknown protein can be cloned,the key amino acids involved in the interaction can also be decided. Drawing protein linkage map is its another application. Being used in drug design, two-hybrid system will have a more practical value in future.