Specificity of the plant innate immune system is often conferred by resistance(R)proteins.Most plant disease resistance (R)proteins contain a series of leucine-rich repeats(LRRs),a nucleotide-binding site(NBS),and a putative amino-terminal signaling domain.They are termed NBS-LRR proteins.The LRRs are mainly involved in recognition,and the amino-terminal domain determines signaling specificity,whereas the NBS domain presumably functions as a molecular switch.During the past years,the most important discoveries are the role of partners in NBS-LRR gene mediated defenses,mounting support for the so-called“guard hypothesis”of R gene function,and providing evidence for intramolecular interactions and intermolecular interactions within NBS- LRR proteins as a mode of signaling regulation.The outcome of these interactions determines whether a plant activates its defense responses. Specificity of the plant innate immune system is often conferred by resistance (R) proteins. Host plant disease resistance (R) proteins contain a series of leucine-rich repeats (LRRs), a nucleotide-binding site (NBS), and a putative amino -terminal signaling domain.They are termed NBS-LRR proteins.The LRRs are mainly involved in recognition, and the amino-terminal domain determines signaling specificity, but the NBS domain presumably functions as a molecular switch. During the past years, the most important discoveries are the role of partners in NBS-LRR gene mediated defenses, mounting support for the so-called “guard hypothesis ” of R gene function, and providing evidence for intramolecular interactions and intermolecular interactions within NBS-LRR proteins as a mode of signaling regulation.The outcome of these contacts decide whether a plant activates its defense responses.