Plants defend themselves against microbial pathogens in several ways. Among the most important of these mechanisms are cytoplasmic nucleotide-binding, leucine-rich repeat (NLR) resistance (R) proteins that are activated by direct or indirect interaction with pathogen-derived effector proteins introduced into the plant cell as part of the pathogenesis process. Effectors that trigger NLR-mediated resistance are known as Avirulence (Avr) proteins. The two major classes of NLR proteins are differentiated by their N-terminal domains being either coiled-coil (CC) or Toll/interleukin-1 receptor (TIR) domains.