Five unrelated avirulence (Avr) gene families have been cloned from flax rust and barley powdery mildew, fungal pathogens that make close contact with living host plant cells using specialized feeding structures called haustoria. Transgenic expression studies indicate Avr proteins are recognized by disease resistance (R) proteins within host cells, which suggests that Avr proteins are transported via an as yet unidentified route from the fungus to the host during infection. Recognition of flax rust AvrL567 proteins is by direct R-Avr protein interaction. Virulence effector functions have been demonstrated for barley powdery mildew Avr proteins Avra10 and Avrk1. Mildew resistance triggered by Avra10 in barley involves association of the cognate barley R protein Mla10 and transcriptional repressor proteins, including HvWRKY2, in the host nucleus. High amplitude defence gene expression has a dual dependence on transcriptional de-repression induced by specific R-Avr protein recognition and additionally, activation signals initiated by host perception of general pathogen molecules.