In gene-for-gene systems, resistance of a plant to a pathogen is due to the ‘recognition’ by plant surveillance system (i.e. plant resistance gene product) of a pathogen avirulence determinant encoded by an avirulence gene to eventually result in triggering of plant immunity. Avirulence proteins (or products) actually are effectors involved in pathogenicity. In viruses, virtually all the proteins can behave as avirulence determinants. In all other cases,avirulence genes are extreme lydiverse,being often species or isolate/strain-specific and rarely have matches in sequence databases. They often are specifically expressed or strongly over-expressed during plant–pathogen interaction and the encoded proteins show the presence of secretion signals and translocation signals (e.g. the Type III secretion signal for bacteria). Avirulence genes seem to be submitted to high-speed diversifying selection allowing the pathogen to diversify its effector repertoire and rapidly escape recognition by the plant resistance gene.