This simple concept stipulates that when investigating by genetic crossing experiments whether the interaction of a pathogen with its host plant is compatible (+) or incompatible (-), segregation analyses will identify in some plant-pathogen systems certain dominant genetic factors in both pathogen and host plant. In such systems, each dominant gene in the pathogen seems to 'match' a dominant gene in the plant host. Because the pathogen gene prevents disease it's been termed 'avirulence' gene (V or Avr) whereby different genotypes or combinations of Avr genes make up respective races. The cognate dominant host gene is called 'resistance' gene (R) and different genotypes or combinations of R genes constitute different cultivars. When an Avr gene is not 'recognized' or 'matched' by its cognate plant R-gene or vice versa, the interaction is compatible and disease ensues. In contrast, 'recognition' or 'matching' kicks on defense mechanisms leading to resistance. The simplest way to envisage this interaction on a molecular level is the elicitor-ligand model in which the Avr gene product, the elicitor, physically interacts with the R gene product. Recent research indicates that Avr gene products in some pathogens might actually be involved in biosynthetic pathways by producing or releasing elicitors rather than acting as one. It is also becoming clear that this concept is not rigid in that a certain Avr gene can be recognized by several Rgenes (in different related plant species), or vice versa.